configure: pass correct cflags to container-based cross compilers

probe_target_compiler returns nonempty $target_cc for installed toolchains
and $container_cross_cc for container-based toolchains.  In both cases
however the flags (coming from $cross_cc_cflags_${target_arch}) must be
in $target_cflags.

Therefore, do not clear them prior to returning from probe_target_compiler.

Reported-by: Taylor Simpson <tsimpson@quicinc.com>
Fixes: 92e288fc ("build: try both native and cross compilers", 2022-07-08)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>