Fix static-link builds: drop the vendored fill_pathname_join_special

The six build-static jobs (ctr, ngc, wii, wiiu, psp, vita) all fail the
same way: 38 "multiple definition" errors, every one of them between
libretro_<platform>.a(file_path.o) and RetroArch's own
libretro-common/file/file_path.o.

Root cause is not the 38 symbols. In a static build RetroArch's
libretro-common objects are on the link line and the core is an archive,
so an archive member is only extracted when it satisfies a symbol that
is still undefined. Every symbol the core's file_path.o defines is
already defined by RetroArch's, with exactly one exception:

  fill_pathname_join_special

RetroArch turned that identifier into a macro alias of
fill_pathname_join(); this vendored copy still had it as a real
function. src/compat/file.c and deps/libretro-common/vfs/
vfs_implementation.c call it, RetroArch no longer defines it, so ld
pulls file_path.o out of the archive to resolve it and the other 38
symbols in that member collide on the way in. One divergent symbol
drags in a whole translation unit.

Sync the vendored copy with upstream: the header gets the macro alias,
the definition goes. The core's file_path.o now exports a strict subset
of RetroArch's, nothing forces its extraction, and the callers resolve
to fill_pathname_join() in both build modes.

The two implementations differed only in fill_pathname_join()'s
`if (s != dir)` guard, which skips the leading copy when the output
aliases the directory. Every call site here passes distinct buffers, so
the guard is never taken and the substitution is a no-op that also makes
the aliasing case safe. A/B oracle over 2640 (dir, path, len)
combinations - empty, relative, absolute, trailing- and leading-slash,
mixed separators, truncating lengths - shows byte-identical output
buffers and identical return values.

Verified by rebuilding the archive against upstream RetroArch's
libretro-common: before, 38 multiple-definition errors matching the CI
log symbol for symbol; after, a clean link. The shared build is
unaffected.

deps/libretro-common/file/file_path.c

@@ -998,35 +998,6 @@ size_t fill_pathname_join(char *s, const char *dir,
    return _len;
 }
 
-/**
- * fill_pathname_join_special:
- * @s                  : output path
- * @dir                : directory. Cannot be identical to @s
- * @path               : path
- * @len                : size of @s
- *
- * Specialized version of fill_pathname_join.
- * Unlike fill_pathname_join(),
- * @dir and @s CANNOT be identical.
- *
- * Joins a directory (@dir) and path (@path) together.
- * Makes sure not to get  two consecutive slashes
- * between directory and path.
- *
- * @return Length of the string copied into @s
- **/
-size_t fill_pathname_join_special(char *s,
-      const char *dir, const char *path, size_t len)
-{
-   size_t _len = strlcpy(s, dir, len);
-
-   if (*s)
-      _len = fill_pathname_slash(s, len);
-
-   _len += strlcpy(s + _len, path, len - _len);
-   return _len;
-}
-
 size_t fill_pathname_join_special_ext(char *s,
       const char *dir,  const char *path,
       const char *last, const char *ext,

deps/libretro-common/include/file/file_path.h

@@ -474,14 +474,19 @@ size_t fill_pathname_join(char *s, const char *dir,
 /**
  * fill_pathname_join_special:
  * @s                  : output path
- * @dir                : directory. Cannot be identical to @s
+ * @dir                : directory
  * @path               : path
  * @len                : size of output path
  *
+ * Macro alias of fill_pathname_join(); the two are identical.
+ * Kept for source compatibility with existing callers in
+ * RetroArch and downstream cores. Each argument expands exactly
+ * once, so arguments with side effects are safe. The distinct
+ * identifier fill_pathname_join_special_ext() is unaffected.
  *
- * Specialized version of fill_pathname_join.
- * Unlike fill_pathname_join(),
- * @dir and @s CANNOT be identical.
+ * @dir and @s may be identical (the join is then performed in
+ * place); buffers that overlap without being identical are
+ * undefined.
  *
  * Joins a directory (@dir) and path (@path) together.
  * Makes sure not to get two consecutive slashes
@@ -492,8 +497,8 @@ size_t fill_pathname_join(char *s, const char *dir,
  *
  * @return Length of the string copied into @s
  **/
-size_t fill_pathname_join_special(char *s,
-      const char *dir, const char *path, size_t len);
+#define fill_pathname_join_special(s, dir, path, len) \
+   fill_pathname_join((s), (dir), (path), (len))
 
 size_t fill_pathname_join_special_ext(char *s,
       const char *dir,  const char *path,