Rename z_Linux_asm.s to z_Linux_asm.S

Summary:
On Unix, a .S file is normally an assembly source which must be
preprocessed with a C preprocessor, while a .s file is "plain" assembly.
The former is handled by the compiler driver (cc), the latter is
directly passed to the assembler binary (as).

Because z_Linux_asm.s is supposed to be preprocessed, rename it to .S,
so it can be automatically picked up correctly by build systems.

Reviewers: AndreyChurbanov, emaste, jlpeyton

Reviewed By: AndreyChurbanov

Subscribers: mgorny, openmp-commits

Differential Revision: https://reviews.llvm.org/D35171

llvm-svn: 307680