[compiler-rt] Produce the right arch suffix for arm baremetal

D98452 introduced a mismatch between clang expectations for
builtin name for baremetal targets on arm. Fix it by
adding a case for baremetal. This now matches the output of
"clang -target armv7m-none-eabi -print-libgcc-file-name \
-rtlib=compiler-rt"

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D113357