SIMD: Don't require definition of HAVE_XSAVE

Currently we fail the compilation via the #error directive if
HAVE_XSAVE isn't defined. This breaks i586 builds since we check
the toolchains SIMD support only on i686 and onward.

Remove the requirement to fix the build on i586.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #13303
Closes #17590