Accept an address as the -e argument

`-e 0x401000` was looked up only as a symbol name, so the link warned
that the entry symbol was undefined and set e_entry to 0. GNU ld and lld
fall back to parsing the argument as a number (decimal, 0x hex or octal)
when no symbol has that name, and so does mold now.

src/chunks/mod.rs

@@ -256,6 +256,13 @@ fn entry_addr<E: Target>(ctx: &Context<E>) -> u64 {
     if let Some(FileId::Obj(_)) = sym.file() {
         return sym.addr(ctx);
     }
+
+    // As in GNU ld and lld, the entry point can also be given as an address
+    // if no symbol has that name.
+    let addr = std::str::from_utf8(sym.name()).ok().and_then(crate::cmdline::parse_c_number);
+    if let Some(addr) = addr {
+        return addr;
+    }
     if !ctx.args.shared {
         warn!("entry symbol is not defined: {sym}");
     }

src/cmdline.rs

@@ -845,7 +845,7 @@ fn parse_hex(opt: &str, value: &str) -> u64 {
 }
 
 /// Parses an integer in C syntax (decimal, `0x` hex or leading-zero octal).
-fn parse_c_number(s: &str) -> Option<u64> {
+pub(crate) fn parse_c_number(s: &str) -> Option<u64> {
     let s = s.trim_start_matches(|c: char| c.is_ascii() && is_space(c as u8));
     let (negative, digits) = if let Some(rest) = s.strip_prefix('-') {
         (true, rest)

tests/entry.sh

@@ -22,3 +22,7 @@ grep "Entry point address:.*0x1000$" $t/log
 $CC -B. -o $t/exe2 -Wl,-e,bar $t/a.o $t/b.o
 readelf -e $t/exe2 > $t/log
 grep "Entry point address:.*0x2000$" $t/log
+
+$CC -B. -o $t/exe3 -Wl,-e,0x3000 $t/a.o $t/b.o
+readelf -e $t/exe3 > $t/log
+grep "Entry point address:.*0x3000$" $t/log