libllvm: promote WithColor and xxhash to SRCS_MIN

The armv6 build failed in CI due to missing symbols (from these two
source files) in the bootstrap Clang.

This affected only armv6 because other Clang-using archs are using LLD
as the bootstrap linker, and thus include SRCS_MIW via LLD_BOOTSTRAP.

Reported by: CI, via lwhsu
Sponsored by: The FreeBSD Foundation