configure: Honour cross-prefix when finding ObjC compiler

Currently when configure picks an ObjectiveC compiler it doesn't pay
attention to the cross-prefix.  This isn't a big deal in practice,
because we only use ObjC on macos and you can't cross-compile to
macos.  But it's a bit inconsistent.

Rearrange the handling of objcc in configure so that we do the
same thing that we do with cc and cxx. This means that the logic
for picking the ObjC compiler goes from:
 if --objcc is specified, use that
 otherwise if clang is available, use that
 otherwise use $cc
to:
 if --objcc is specified, use that
 otherwise if --cross-prefix is specified, use ${cross_prefix}clang
 otherwise if clang is available, use that
 otherwise use $cc

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1185


Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230418161554.744834-1-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>