Make gdb.threads/step-over-trips-on-watchpoint.exp effective on !x86

Make gdb.threads/step-over-trips-on-watchpoint.exp effective on !x86

This test is currently failing like this on (at least) PPC64 and s390x:

 FAIL: gdb.threads/step-over-trips-on-watchpoint.exp: no thread-specific bp: step: step
 FAIL: gdb.threads/step-over-trips-on-watchpoint.exp: no thread-specific bp: next: next
 FAIL: gdb.threads/step-over-trips-on-watchpoint.exp: with thread-specific bp: step: step
 FAIL: gdb.threads/step-over-trips-on-watchpoint.exp: with thread-specific bp: next: next

gdb.log:

 (gdb) PASS: gdb.threads/step-over-trips-on-watchpoint.exp: no thread-specific bp: step: set scheduler-locking off
 step
 wait_threads () at ../../../src/gdb/testsuite/gdb.threads/step-over-trips-on-watchpoint.c:49
 49        return 1; /* in wait_threads */
 (gdb) FAIL: gdb.threads/step-over-trips-on-watchpoint.exp: no thread-specific bp: step: step

The problem is that the test assumes that both the "watch_me = 1;" and
the "other = 1;" lines compile to a single instruction each, which
happens to be true on x86, but no necessarily true everywhere else.
The result is that the test doesn't really test what it wants to test.

Fix it by looking for the instruction that triggers the watchpoint.

gdb/ChangeLog:
2015-04-10  Pedro Alves  <palves@redhat.com>

	* gdb.threads/step-over-trips-on-watchpoint.c (child_function):
	Remove comment.
	* gdb.threads/step-over-trips-on-watchpoint.exp (do_test): Find
	both the address of the instruction that triggers the watchpoint
	and the address of the instruction immediately after, and use
	those addresses for the test.  Fix comment.