[gdb/testsuite] Handle for loop initial decl with gcc 4.8.5

[gdb/testsuite] Handle for loop initial decl with gcc 4.8.5

When running test-case gdb.threads/schedlock-thread-exit.exp on a system with
system compiler gcc 4.8.5, I run into:
...
src/gdb/testsuite/gdb.threads/schedlock-thread-exit.c:33:3: error: \
  'for' loop initial declarations are only allowed in C99 mode
...

Fix this by:
- using -std=c99, or
- using -std=gnu99, in case that's required, or
- in the case of the jit test-cases, rewriting the for loops.

Tested on x86_64-linux, both with gcc 4.8.5 and gcc 7.5.0.