i686: Add missing IS_IN (libc) guards to vectorized strcspn

Since commit d957c4d3 (i386: Compile
rtld-*.os with -mno-sse -mno-mmx -mfpmath=387), vector intrinsics can
no longer be used in ld.so, even if the compiled code never makes it
into the final ld.so link.  This commit adds the missing IS_IN (libc)
guard to the SSE 4.2 strcspn implementation, so that it can be used from
ld.so in the future.