Also in preparation for handling more binaries from the cross binutils,
support an option --cross-prefix-ARCH.  All cross_cc_* defaults are
replaced with cross_prefix_*; the cross_cc_* fallbacks are extended
to the cross-compilation prefix, but the compiler fallbacks remain
as well.  This way, for example, --cross-cc-arm=arm-linux-gnueabihf-clang
also applies to armeb binaries.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220517092616.1272238-11-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220527153603.887929-22-alex.bennee@linaro.org>