[sanitizer_common][test] Handle missing REG_STARTEND in Posix/regex_startend.cpp

As reported in D96348 <https://reviews.llvm.org/D96348>, the
`Posix/regex_startend.cpp` test `FAIL`s on Solaris because
`REG_STARTEND` isn't defined.  It's a BSD extension not present everywhere.
E.g. AIX doesn't have it, too.

Fixed by wrapping the test in `#ifdef REG_STARTEND`.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D98425