[Bug libstdc++/66414] string::find ten times slower than strstr

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66414

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Although we can only do that for the overload that assumes null-terminated
input, not all cases that basic_string::find supports.

We can use the GNU memmem extension for the general case, but that's not
standard.