Driver: unify compiler-rt component selection

Unify the component handling for compiler-rt.  The components are regularly
named, built up from:

  ${LIBRARY_PREFIX}clang_rt.${component}-${arch}[-${environment}]${LIBRARY_SUFFIX}

Unify the handling for all the various components, into a single path to link
against the various components in a number of places.  This reduces duplication
of the clang_rt library name construction logic.

llvm-svn: 225013