qemu/bswap: Use compiler __builtin_bswap() on FreeBSD

qemu/bswap: Use compiler __builtin_bswap() on FreeBSD

Since commit efc6c070aca ("configure: Add a test for the minimum
compiler version") the minimum compiler version required for GCC
is 4.8, which supports __builtin_bswap().
Remove the FreeBSD specific ifdef'ry.

This reverts commit de03c3164accc21311c39327601fcdd95da301f3
("bswap: Fix build on FreeBSD 10.0").

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230111163147.71761-6-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>