Use __builtin___stpncpy_chk when available

Use __builtin___stpncpy_chk when available

The builtin has been available in gcc since 4.7.0 and in clang since
2.6.  This fixes stpncpy fortification with clang since it does a
better job of plugging in __stpncpy_chk in the right place than the
header hackery.

This has been tested by building and running all tests with gcc 10.2.1
and also with clang tip as of a few days ago (just the tests in debug/
since running all tests don't work with clang at the moment) to make
sure that both compilers pass the stpncpy tests.