rename ARM's libunwind.S to to avoid conflict with llvm libunwind

llvm libunwind includes a libunwind.cpp, but on ARM libunwind.S is found
first in .PATH. Rename the latter one, since it is not going to be
updated again.

Reviewed by: andrew
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7162