[libunwind] Export the weak alias in Mach-O

[phosek]

Summary

This is not necessary for ELF since .globl and .weak are mutually
exclusive, but is necessary for Mach-O otherwise the symbol isn't
visible externally.

[phosek]

This addresses the issue that was introduced in r357640 and detected by our 2-stage LTO builder.

[mcgrathr]

Lgtm

libunwind/src/assembly.h

 
 #if defined(__APPLE__)
 
 #define SYMBOL_IS_FUNC(name)
 #define EXPORT_SYMBOL(name)
 #define HIDDEN_SYMBOL(name) .private_extern name
 #define WEAK_SYMBOL(name) .weak_reference name
 #define WEAK_ALIAS(name, aliasname)                                            \
+  .globl SYMBOL_NAME(aliasname) SEPARATOR                                      \
   WEAK_SYMBOL(aliasname) SEPARATOR                                             \
   SYMBOL_NAME(aliasname) = SYMBOL_NAME(name)
 
 #define NO_EXEC_STACK_DIRECTIVE
 
 #elif defined(__ELF__)
 
 #if defined(__arm__)