configure: handle host compiler in probe_target_compiler

configure: handle host compiler in probe_target_compiler

In preparation for handling more binaries than just cc, handle
the case of "probe_target_compiler $cpu" directly in the function,
setting the target_* variables based on the ones that are used to
build QEMU.  The clang check also needs to be moved after this
fallback.

Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-Id: <20220517092616.1272238[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Message-Id: <20220527153603[email protected]>