[gdb/testsuite] Fix gdb.base/endianity.exp with gcc-4.8

[gdb/testsuite] Fix gdb.base/endianity.exp with gcc-4.8

When running test-case gdb.base/endianity.exp using gcc-4.8, we get:
...
(gdb) x/x &o.v^M
0x7fffffffd120: 0x00000004^M
(gdb) XFAIL: gdb.base/endianity.exp: x/x &o.v
x/xh &o.w^M
0x7fffffffd124: 0x0003^M
(gdb) FAIL: gdb.base/endianity.exp: x/xh &o.w
...

The gcc 4.8 compiler does not support the scalar_storage_order attribute, so
the testcase is compiled without that attribute, and the expected results are
different.

Fix this by rather than xfailing, skipping the tests if the compiler does not
support the scalar_storage_order attribute.

Tested on x86_64-linux, with gcc-4.8, gcc-7, and clang-10.

gdb/testsuite/ChangeLog:

2020-12-13  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26953
	* gdb.base/endianity.exp: Skip tests requiring scalar_storage_order
	attribute support if compiler doesn't support it.