Revert "[msan] Intercept qsort, qsort_r."

This reverts commit 7a9ebe95, and
dependent commit 54c52242, which
disables qsort interception for some iOS platforms.

After this change, the -Nolibc sanitizer common test binary crashes on
startup on my regular Linux workstation, as well as on our bots:
https://ci.chromium.org/p/chromium/builders/try/linux_upload_clang/740

 ********************
  Failing Tests (1):
       SanitizerCommon-Unit ::
       ./Sanitizer-x86_64-Test/SanitizerCommon.NolibcMain

Loading it up in gdb shows that it crashes during relocation processing,
which suggests that some glibc loader versions do not support the
THREADLOCAL data added in this interceptor.