BaseTools: Factorize GCC flags

BaseTools: Factorize GCC flags

GCC48_ALL_CC_FLAGS has no dependency on GCC_ALL_CC_FLAGS.
By definition, there should be such dependency.

The outcomes of this patch is that GCC48_ALL_CC_FLAGS and
other dependent configurations will inherit from the
additional "-Os" flag.
The "-Os" flag optimizes a build in size, not breaking any
build. In a gcc command line, the last optimization flag
has precedence. This means that this "-Os" flag will be
overriden by a more specific optimization configuration,
provided that this more specific flag is appended at the
end of the CC_FLAGS.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Suggested-by: Tomas Pilar <Tomas.Pilar@arm.com>
Reviewed-by: Bob Feng<bob.c.feng@intel.com>