[libunwind] Set LIBUNWIND_ASM_SOURCES to the ASM source language from C

I believe this is an oversight from the import of libunwind into its own
library from libc++abi.

In libc++abi, these files had the .s suffix, which indicates that the file
is a preprocessed assembly source file. This caused problems because the
files rely upon preprocessors to guard target-specific blocks.

To fix this, the CMakeLists file marked these files as C so that the
preprocessor would be run over them, but then the compiler would correctly
identify the files as assembly and compile them as such.

When imported to libunwind, these files were (correctly) renamed with .S
suffixes, which are non-preprocessed assembly. Thus, we no longer need the
C language property.

The benefit here is that the files can now benefit from CMAKE_ASM_FLAGS
rather than CMAKE_C_FLAGS.

Patch By: JamesNagurne

Differential Revision: https://reviews.llvm.org/D72952