optionrom: do not rely on compiler's bswap optimization

optionrom: do not rely on compiler's bswap optimization

Recent compilers can detect and inline manually-written bswap code,
but GCC 4.2.1 (the last GPLv2 version) cannot and generates really
awful code.  Depending on how the compiler is configured, it might
also not want to generate bswap because it was not in i386.  Using
asm is fine because TCG knows about bswap and all processors with
virtualization extensions also do.

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