[BOLT][test] Fix AArch64 cross-platform tests

Use target-specific flags for building AArch64 non-runnable tests.

Reviewed By: yota9

Differential Revision: https://reviews.llvm.org/D122520

bolt/test/AArch64/double_jump.cpp

@@ -52,4 +52,4 @@ unsigned long foo(unsigned long count) {
   return count;
 }
 
-int main(int argc, const char *argv[]) { return foo(38); }
+extern "C" int _start() { return foo(38); }

bolt/test/AArch64/lit.local.cfg

@@ -1,2 +1,7 @@
 if config.host_arch not in ['aarch64']:
     config.unsupported = True
+
+config.substitutions.insert(
+    0, ('%cflags',
+        '%cflags --target=aarch64-pc-linux -nostartfiles -nostdlib -fuse-ld=lld'
+        ' -Wl,--unresolved-symbols=ignore-all'))

bolt/test/AArch64/tailcall_traps.s

@@ -17,12 +17,12 @@
 
   .text
   .align 4
-  .global main
-  .type main, %function
-main:
+  .global _start
+  .type _start, %function
+_start:
   nop
   ret
-  .size main, .-main
+  .size _start, .-_start
 
   .global foo
   .type foo, %function