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

With test-case gdb.base/nodebug.exp I run into:
...
gdb compile failed, gdb.base/nodebug.c: In function 'multf_noproto':
gdb.base/nodebug.c:63:1: warning: old-style function definition \
  [-Wold-style-definition]
   63 | multf_noproto (v1, v2)
      | ^~~~~~~~~~~~~
...

Fix this using -std=c99.

Tested on x86_64-linux.

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