cmake: Only relativize non-relative toolchain file paths.
cmake: Only relativize non-relative toolchain file paths.

BUG=aomedia:2164

Change-Id: Idf65a7e5c280c49c29203b5b2318c5099ba90c16

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom