Only include PPC-specific files in PPC builds

From acm in WT-2629:

can we also make it so this file is not compiled at all unless we are actually on ppc64le? SCons treats assembly files in odd ways and it is causing some trouble for me in other areas of the runtime hardening project. I'd prefer not to need to investigate a workaround.

This will mean either removing these files from dist/filelist and adding them as special cases in build_posix/Make.base and src/third_party/wiredtiger/SConscript in MongoDB, or adding extra information to dist/filelist about conditional files, like we do in Make.subdirs.