libs/libc/string: add memmem

[Cynerd]

This function is the GNU extension.

Summary

Impact

Testing

Tested on custom SAME70N21 based board.

[xiaoxiang781216]

@Cynerd let's squash these patch into one.

[pkarashchenko]

Please fix build error

string/lib_memmem.c: In function 'memmem':
Error: string/lib_memmem.c:65:24: error: return discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
   65 |               return h + i;