[CMake] Don't parse target triple except for arch

compiler-rt CMake build currently tries to parse the triple and then
put it back together, but doing so inherently tricky, and doing so
from CMake is just crazy and currently doesn't handle triples that
have more than three components. Fortunatelly, the CMake really only
needs the architecture part, which is typically the first component,
to construct variants for other architectures. This means we can keep
the rest of the triple as is and avoid the parsing altogether.

Differential Revision: https://reviews.llvm.org/D50548

llvm-svn: 339701