Linux x86 SIMD: factor out unneeded kernel dependencies

Cleanup the kernel SIMD code by removing kernel dependencies.

• Replace XSTATE_XSAVE with our own XSAVE implementation for all kernels not exporting kernel_fpu{begin,end}(), see #13059
• Replace union fpregs_state by a uint8_t * buffer and get the size of the buffer from the hardware via the CPUID instruction
• Replace kernels xgetbv() by our own implementation which was already there for userspace.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #13102