[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
GitOrigin-RevId: ee676e2f6f84bc89354ede366939f038acf14763
1 file changed