[unwinder] Tell the assembler that functions are functions so that the linker...

[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'.

llvm-svn: 214573