This was added without explanation by commit v2.8.2~1138 (Add initial XL
C compiler flags for safer builds, 2009-09-30). It is not consistent
with our default C++ flags for XL and disables several optimizations, so
drop it from our default flags for C.
Suggested-by: Todd Gamblin <tgamblin@llnl.gov>
Previously, it was inconsistent in that some platforms/compilers
had this flag for the RelWithDebInfo configuration and some didn't.
This fixes issue #11366.