bsls_platform: Updated Clang GCC version selection

Clang, by default, defines pre-processor macros that indicate the
compiler is GCC and that it is specifically version 4.2.1.  BDE
already previously overrode this selection to indicate version
4.4.0 because Clang supports features found in the latest version(s)
of GCC.  This change makes Clang >= 3.3 mimic GCC 4.8.1, as both
compilers were the first to have full C++11 support.  This will make
Clang >= 3.3 compilation correctly include GCC atomic intrinsics that
are present only in GCC >= 4.7.