gdb.reverse/time-reverse.exp: test both time syscall and C time function

gdb.reverse/time-reverse.exp: test both time syscall and C time function

Instead of only testing this on systems that have a SYS_time syscall,
test it everywhere using the time(2) C function, and in addition, run
the tests again using the SYS_time syscall.

The C variant ensures that if some platform uses some syscall we are
not aware of yet, we'll still exercise it, and likely fail, at which
point we should teach GDB about the syscall.

The explicit syscall variant is useful on platforms where the C
function does not call a syscall at all by default, e.g., on some
systems the C time function wraps an implementation provided by the
vDSO.

Approved-By: Tom de Vries <tdevries@suse.de>
Change-Id: Id4b755d76577d02c46b8acbfa249d9c31b587633