Tin Huynh authored Jul 16, 2020 and Brad King committed Jul 22, 2020

Since commit 9b97cb55 (PGI: Add language standards for PGI,
2017-05-01, v3.9.0-rc1~174^2), we have passed the `-A` flag to
the PGI C++ compiler when specifying a C++ standard flag with
compiler extensions turned off.  The flag is not meant for that.
The PGI C++ standard flags do not turn extensions on by default
and have a separate `--gnu_extensions` flag for that which we
already use when CXX_EXTENSIONS is ON.  Simply drop the `-A` flag.

Fixes: #20997