RTS: Fix compile on powerpc64 ELF v1

Cabal does not know about the different ABIs for powerpc64 and compiles
StgCRunAsm.S unconditionally. The old make-based build system excluded
this file from the build and it was OK to signal an error when it was
compiled accidentally.

With this patch we compile StgCRunAsm.S to an empty file, which fixes
the build.

Fixes #25700