[libunwind] Export the weak alias in Mach-O

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.

Differential Revision: https://reviews.llvm.org/D60245

llvm-svn: 357671

libunwind/src/assembly.h

@@ -47,6 +47,7 @@
 #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)