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

[maksfb]

Summary

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

[yota9]

Overall LGTM, but the same changes must be applied to x86 folder of course

bolt/test/AArch64/double_jump.cpp

55

Although it will probably built, but I sugget _start here

[yota9]

BTW we may want to use -Wl,--unresolved-symbols=ignore-all option to minimize the test changes and overcome problem with stdlib/starfiles. It might be more problematic to change all x86 test to use these flags

[maksfb]

Address feedback.

[maksfb]

In D122520#3409609, @yota9 wrote:

Overall LGTM, but the same changes must be applied to x86 folder of course

Yes, this one is AArch64-only.

[yota9]

LGTM Thank you!

[maksfb]

Rebase.

bolt/test/AArch64/double_jump.cpp

asm volatile("     cmp %0,#0\n"
                "     b .L6\n"
                ".L6: b bar\n"
                :
                : "r"(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

 if 'AArch64' not in config.root.targets:
     config.unsupported = True
+
+config.substitutions.insert(
+  0, ('%cflags',
+      '%cflags --target=aarch64-pc-linux -nostartfiles -nostdlib -fuse-ld=lld'))

bolt/test/AArch64/tailcall_traps.s

 # CHECK:  End of Function "foo"
 
 # CHECK:  Binary Function "bar"
 # CHECK:        b     foo # TAILCALL
 # CHECK:  End of Function "bar"
 
   .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
 foo:
   br x0
   .size foo, .-foo
 
   .global bar
   .type bar, %function
 bar:
   b foo
   .size bar, .-bar