SystemInformation: Replace __GNUG__ with __GNUC__

Commit 857bda17 (SystemInformation: Report more information, 2012-05-29)
added use of __GNUG__ which is equivalent to __GNUC__ && __cplusplus.
Since this is a C++ source file and we use __GNUC__ elsewhere it is more
consistent and less redundant to use the latter.

Change-Id: I6428e2601310dd35c14ac643dcec199b01977842