[gdb/testsuite] Add xfails for PR gcc/90232

[gdb/testsuite] Add xfails for PR gcc/90232

With target board debug-types, we have these FAILs:
...
FAIL: gdb.guile/scm-symtab.exp: test simple_struct in static symbols
FAIL: gdb.python/py-symtab.exp: test simple_struct in static symbols
...
due to PR gcc/90232, as explained in commit 15cd93d05e8 "[gdb/symtab] Handle
struct decl with DW_AT_signature".

Marks these as XFAILs.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-29  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (debug_types): New proc.
	* gdb.guile/scm-symtab.exp: Add xfail for PR gcc/90232.
	* gdb.python/py-symtab.exp: Same.