[gdb/testsuite] Add KFAILs for PR symtab/24549

[gdb/testsuite] Add KFAILs for PR symtab/24549

When an executable contains an index such as a .gdb_index or .debug_names
section, gdb ignores the DW_AT_subprogram attribute.

This problem has been filed as PR symtab/24549.

Add KFAILs for this PR in test-cases gdb.dwarf2/main-subprogram.exp and
gdb.fortran/mixed-lang-stack.exp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-02-05  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/main-subprogram.exp: Add KFAIL for PR symtab/24549.
	* gdb.fortran/mixed-lang-stack.exp: Same.