[gdb/testsuite] Introduce supports_scalar_storage_order_attribute

Introduce support test procs:
- supports_scalar_storage_order_attribute, and
- supports_gnuc
and use them in test-case gdb.base/endianity.exp.

Tested on x86_64-linux with gcc-7.5.0, gcc-4.8.5, and clang 10.0.1.

gdb/testsuite/ChangeLog:

2020-12-19  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (supports_scalar_storage_order_attribute)
	(supports_gnuc): New proc.
	* gdb.base/endianity.exp: Define TEST_SSO.  Eliminate
	test_compiler_info calls.  Add unsupported message.
	* gdb.base/endianity.c: Use TEST_SSO.