For arm and armv6, only enable LLVM target support for arm by default,
to shrink libllvm.a.

This is a workaround for "relocation truncated to fit" errors with BFD
ld 2.17.50 on arm and armv6, when linking executables against it.

The required range extensions are not yet supported by this very old
version of BFD ld. When arm and armv6 userland can be successfully
linked by lld, this workaround can be removed.