[gdb/testsuite] Require syscall time in gdb.reverse/time-reverse.exp

[gdb/testsuite] Require syscall time in gdb.reverse/time-reverse.exp

On aarch64-linux, I run into:
...
Running gdb.reverse/time-reverse.exp ...
gdb compile failed, gdb.reverse/time-reverse.c: In function 'main':
gdb.reverse/time-reverse.c:39:12: error: 'SYS_time' undeclared \
  (first use in this function); did you mean 'SYS_times'?
   syscall (SYS_time, &time_global);
            ^~~~~~~~
            SYS_times
gdb.reverse/time-reverse.c:39:12: note: each undeclared identifier is \
  reported only once for each function it appears in
UNTESTED: gdb.reverse/time-reverse.exp: failed to prepare
...

Fix this by adding a new proc have_syscall, and requiring syscall time, such
that we have instead:
...
UNSUPPORTED: gdb.reverse/time-reverse.exp: require failed: \
  expr [have_syscall time]
...

Tested on x86_64-linux and aarch64-linux.