MdePkg/BaseLib.h: state preprocessing conditions in comments after #endifs

"#endif" preprocessing directives near the top of "BaseLib.h" helpfully
repeat the preprocessing conditions from their matching "#if", "#ifdef",
and "#ifndef" directives. This practice has been less followed recently;
supplement the missing comments.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=866
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>