[gdb/testsuite] Require gcc >= 5 in gdb.linespec/cpls-abi-tag.exp

When running test-case gdb.linespec/cpls-abi-tag.exp with gcc 4.8.4, we run
into:
...
cpls-abi-tag.cc:71:26: error: ‘abi_tag’ attribute applied to non-function ‘s’
 ABI3 test_abi_tag_struct s;
                          ^
...

The test-case is supported starting gcc 5.

Fix this by requiring gcc >= 5, if a gcc compiler is used.

Tested on x86_64-linux.

gdb/testsuite/gdb.linespec/cpls-abi-tag.exp

@@ -21,6 +21,14 @@ load_lib completion-support.exp
 
 standard_testfile cpls-abi-tag.cc
 
+if { [test_compiler_info gcc-*] } {
+    # With earlier gcc versions we run into:
+    #   cpls-abi-tag.cc:71:26: error:
+    #     ‘abi_tag’ attribute applied to non-function ‘s’
+    # See gcc PR65046.
+    require {expr [gcc_major_version] >= 5}
+}
+
 if {[prepare_for_testing "failed to prepare" $testfile \
 	 [list $srcfile] {c++ debug}]} {
     return -1