Clean up SSE4.2 support, and no longer use asm fallback or gcc builtin.

Clean up SSE4.2 support, and no longer use asm fallback or gcc builtin.

Defines changing meaning:
X86_SSE42 used to mean the compiler supports crc asm fallback.
X86_SSE42_CRC_INTRIN used to mean compiler supports SSE4.2 intrinsics.

X86_SSE42 now means compiler supports SSE4.2 intrinsics.

This therefore also fixes the adler32_sse42 checks, since those were depending
on SSE4.2 intrinsics but was mistakenly checking the X86_SSE42 define.
Now the X86_SSE42 define actually means what it appears to.