[gdb/testsuite] Use -std=gnu99 for gdb.server/attach-flag.exp

[gdb/testsuite] Use -std=gnu99 for gdb.server/attach-flag.exp

When using a compiler defaulting to -std=gnu90, we run into:
...
Running gdb.server/attach-flag.exp ...
gdb compile failed, attach-flag.c: In function 'main':
attach-flag.c:22:3: error: 'for' loop initial declarations are only allowed \
  in C99 or C11 mode
   for (int i = 0; i < NTHREADS; i++)
   ^~~
attach-flag.c:22:3: note: use option -std=c99, -std=gnu99, -std=c11 or \
  -std=gnu11 to compile your code
...

Fix this by using -std=gnu99.

Tested on x86_64-linux.