crt: Mark __guard_dispatch_icall_dummy as global

In LLVM LTO builds with cfguard enabled, the non-extern
__guard_dispatch_icall_dummy can't pose as replacement for the
undefined symbol reference to an extern __guard_dispatch_icall_dummy.

Signed-off-by: Martin Storsjö <martin@martin.st>

mingw-w64-crt/cfguard/mingw_cfguard_support.c

@@ -20,6 +20,7 @@ static void __guard_check_icall_dummy(void) {}
 // When CFGuard is not active, directly tail-call the target address, which
 // is passed via %rax.
 __asm__(
+    ".globl __guard_dispatch_icall_dummy\n"
     "__guard_dispatch_icall_dummy:\n"
     "    jmp *%rax\n"
 );