Fix qsort() interceptor for FreeBSD

Fix qsort() interceptor for FreeBSD

When the FreeBSD qsort() implementation recurses, it does so using an
interposable function call, so we end up calling the interceptor again
and set the saved comparator to wrapped_qsort_compar. This results in an
infinite loop and a eventually a stack overflow since wrapped_qsort_compar
ends up calling itself. This means that ASAN is completely broken on
FreeBSD for programs that call qsort(). I found this while running
check-all on a FreeBSD system a ASAN-instrumented LLVM.

Fix this by checking whether we are recursing inside qsort before writing
to qsort_compar. The same bug exists in the qsort_r interceptor, so use the
same approach there. I did not test the latter since the qsort_r function
signature does not match and therefore it's not intercepted on FreeBSD/macOS.

Fixes https://llvm.org/PR46832

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D84509