Fix hardware watchpoints in replay mode

Changes introduced by commit 9e8915c6
caused a regression that meant hardware watchpoint stops would not
trigger in reverse execution or replay mode.  This was documented in
PR breakpoints/21969.
The problem is that record_check_stopped_by_breakpoint always overwrites
record_full_stop_reason, thus loosing the TARGET_STOPPED_BY_WATCHPOINT
value which would be checked afterwards.

This commit fixes that by not overwriting the stop-reason in
record_full_stop_reason if we're not stopped at a breakpoint.

And the test for hw watchpoints in gdb.reverse/watch-reverse.exp actually
tested sw watchpoints again, since "set can-use-hw-watchpoints 1"
doesn't convert enabled watchpoints to use hardware.
This is fixed by disabling said watchpoint while enabling hw watchpoints.
The same is not done for gdb.reverse/watch-precsave.exp, since it's not
possible to use hw watchpoints in restored recordings anyways.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=21969


Approved-by: Guinevere Larsen <blarsen@redhat.com>