gcc: add hidden symbol for defaults

gcc: add hidden symbol for defaults

Some architectures, f.e. Blackfin doesn't support to configure GCC with
--with-cpu to set some CPU specific default CFLAGS (-mcpu=foo). Use a
hidden config symbol to give a hint which architecture supports it,
otherwise add defaults to toolchain wrapper for internal toolchains.

Idea from Thomas Petazzoni.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
[Thomas:
 - simplify the Config.in logic with just one option named
   BR2_GCC_ARCH_HAS_CONFIGURABLE_DEFAULTS, defined in package/gcc in one
   place.
 - improve the organization of the code and name of variables.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>