Don't abort when calling qfind_first_byte_of_sse42 without compiler support for SSE 4.2

Don't abort when calling qfind_first_byte_of_sse42 without compiler support for SSE 4.2

Summary:
If Folly was compiled without support for SSE 4.2, but this function ends up getting called, we'd abort due to the check failing.
To solve this, just don't do the check and fall back to the non-sse version.

This is reasonable because it means that the code calling `qfind_first_byte_of_sse42` only needs to care whether there is CPU support for SSE 4.2, not whether we were able to actually implement the search with SSE 4.2 based on compiler support.

Reviewed By: yfeldblum

Differential Revision: D4096645

fbshipit-source-id: d26a791c09f61595e40c84c47a7a2596b9b69c1e