[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:

2020-12-10  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.

---
 gdb/testsuite/gdb.base/endianity.exp | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/gdb/testsuite/gdb.base/endianity.exp b/gdb/testsuite/gdb.base/endianity.exp
index 2fa9ed3bf1..4520799d04 100644
--- a/gdb/testsuite/gdb.base/endianity.exp
+++ b/gdb/testsuite/gdb.base/endianity.exp
@@ -34,12 +34,15 @@ gdb_test "print o.x = 2" "= 2"
 gdb_test "print o.f = 1.5" "= 1.5"
 gdb_test "print o.d = -23.125" "= -23.125"
 
-# scalar_storage_order requires gcc >= 6
+gdb_test "print o" "= {v = 4, w = 3, x = 2, f = 1.5, cplx = 1.25 \\+ 7.25i, d = -23.125}" \
+    "print o after assignment"
+
 if { ([test_compiler_info {gcc-[0-5]-*}] || ![test_compiler_info gcc*]) } {
-  setup_xfail "*-*-*"
+    # The rest of the testcase requires Scalar Storage Order support.
+    # This compiler does not support it, so skip the rest.
+    return -1
 }
+
 gdb_test "x/x &o.v" "0x04000000"
 gdb_test "x/xh &o.w" "0x0300"
 
-gdb_test "print o" "= {v = 4, w = 3, x = 2, f = 1.5, cplx = 1.25 \\+ 7.25i, d = -23.125}" \
-    "print o after assignment"