[gdb/testsuite] Use -std=c99 in gdb.base/callfuncs.exp

In test-case gdb.base/callfuncs.exp I run into:
...
gdb compile failed, gdb.base/callfuncs.c: In function 't_func_values':
gdb.base/callfuncs.c:611:12: error: too many arguments to function \
  'func_arg1'; expected 0, have 2
  611 |   return ((*func_arg1) (5,5)  == (*func_val1) (5,5)
      |           ~^~~~~~~~~~~  ~
...

Fix this by using -std=c99.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR testsuite/32756
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32756