[unwinder] Tell the assembler that functions are functions so that the linker knows how to link them.

This fixes an interworking problem when the unwinder/libcxxabi is built for
Thumb. When unw_getcontext is not marked as a function, 'bl' is used for the
branch instead of 'bx'.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@214573 91177308-0d34-0410-b5e6-96231b3b80d8