[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

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@357671 91177308-0d34-0410-b5e6-96231b3b80d8