qemu/bswap: Use compiler __builtin_bswap() on NetBSD

qemu/bswap: Use compiler __builtin_bswap() on NetBSD

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 NetBSD specific ifdef'ry.

This reverts commit 1360677cfe3ca8f945fa1de77823df21a77e4500
("makes NetBSD use the native bswap functions").

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230111163147.71761-7-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>