um: fix and optimize xor select template for CONFIG64 and timetravel mode

Due to dropped inclusion of asm-generic/xor.h, xor_block_8regs symbol is
missing with CONFIG64 and break compilation, as the asm/xor_64.h also did
not include it. The patch recreate the logic from arch/x86, which check
whether AVX is available and add fallbacks for 32bit and 64bit config of
um.

A very minor additional "fix" is, the return of the macro parameter
instead of NULL, as this is the original intent of the macro, but
this does not change the actual behavior.

Fixes: c0ecca66

 ("um: enable the use of optimized xor routines in UML")
Signed-off-by: Benjamin Beichler <benjamin.beichler@uni-rostock.de>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>