Prepare for powerpc-on-clang by deleting hard-coded tests for architecture
as a stand-in for "are we running on gcc". (PR 239181)

As a side-effect, adding the stanza

MAKE_ENV= LLVM_CONFIG=${LOCALBASE}/bin/llvm-config${LLVM_DEFAULT}

obviates the need for hardcoding definitions for CC, CXX, and CPP
(later linimon testing).

PR: 239181
Approved by: danfe (maintainer-timeout, > 2 months)