powerpc64/csu: Rename dummy asm file to force use of 'cc'

Implicit make rules build .S asm files with the compiler, rather than the
assembler. r356889 removed GNU as from the build for powerpc targets,
causing '.s' asm files to fail to build, due to a missing 'as'. Rename the
one dummy asm file we have to a '.S' to force the implicit rules to build
with the compiler rather than the assembler.

Reported by: Francis Little