tilegx: provide optimized strnlen, strstr, and strcasestr

strnlen() is based on the existing tile strlen() with length
checking added.  It speeds up by up to 5x, but on average across
the benchtest corpus by around 35%.  No regressions are seen.

strstr() does 8-byte aligned loads and compares using a 2-byte
filter on the first two bytes of the needle and then testing
the remaining bytes in needle using memcmp().  It speeds up
about 5x in the best case (for "found" needles), about 2x looking
at benchtest as a whole, with some slowdowns as much as 45%.
on a few cases (including the "fail" case for 128KB search).

strcasestr() is based on strstr() but uses a SIMD tolower
routine to convert 8-bytes to lower case in 5 instructions.
It also uses a 2-byte filter and then strncasecmp() for the
remaining bytes.  strncasecmp() is not optimized for SIMD, so
there is futher room for improvement.  However, it is still up
to 16x faster for "found" needles, averaging 2x faster on the
whole corpus of benchtests.  It does slow down by up to 35%
on a few cases, similarly to strstr().

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