toolchain: xcc: fix build with non-clang xcc versions

toolchain/xcc.h includes toolchain/gcc.h in some configurations
and this will lead to build errors as __in_section_unique_named()
and friends are defined multiple times.

Add ifdef guards to main definitions in gcc.h.

Fixes: 7e32b206 ("toolchain: xcc: Avoid undefs")
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>