BaseTools/Source/C: split "-O2" to BUILD_OPTFLAGS

BaseTools/Source/C: split "-O2" to BUILD_OPTFLAGS

The option "-O2" is not a preprocessor flag, but a code generation
(compilation) flag. Move it from BUILD_CPPFLAGS to BUILD_CFLAGS and
BUILD_CXXFLAGS.

Because "VfrCompile/GNUmakefile" uses "-O2" through BUILD_CPPFLAGS, and
because it doesn't use BUILD_CXXFLAGS, we have to introduce BUILD_OPTFLAGS
separately, so that "VfrCompile/GNUmakefile" can continue using just this
flag.

This patch doesn't change behavior.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1540244
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>