configure: enable cross-compilation of optionrom

configure: enable cross-compilation of optionrom

While container-based cross compilers are not supported, this already makes
it possible to build x86 optionroms on any machine that has an installation
of GCC and binutils for 32- or 64-bit x86.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220517092616.1272238-15-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220527153603.887929-26-alex.bennee@linaro.org>