GDB/testsuite: Add/correct gdb.reverse timeout tweaks

GDB/testsuite: Add/correct gdb.reverse timeout tweaks

There are three cases in two scripts in the gdb.reverse subset that
take a particularly long time.  Two of them are already attempted to
take care of by extending the timeout from the default.  The remaining
one has no precautions taken.  The timeout extension is ineffective
though, it is done by adding a constant rather than by scaling and as
a result while it may work for target boards that get satisfied with
the detault test timeout of 10s, it does not serve its purpose for
slower ones.

Here are indicative samples of execution times (in seconds) observed
for these cases respectively, for an ARMv7 Panda board running Linux
and a `-march=armv5te' multilib:

PASS: gdb.reverse/sigall-reverse.exp: continue to signal exit
elapsed: 385
PASS: gdb.reverse/until-precsave.exp: run to end of main
elapsed: 4440
PASS: gdb.reverse/until-precsave.exp: save process recfile
elapsed: 965

for the same board and a `-mthumb -march=armv5te' multilib:

PASS: gdb.reverse/sigall-reverse.exp: continue to signal exit
elapsed: 465
PASS: gdb.reverse/until-precsave.exp: run to end of main
elapsed: 4191
PASS: gdb.reverse/until-precsave.exp: save process recfile
elapsed: 669

and for QEMU in the system emulation mode and a `-march=armv4t'
multilib:

PASS: gdb.reverse/sigall-reverse.exp: continue to signal exit
elapsed: 45
PASS: gdb.reverse/until-precsave.exp: run to end of main
elapsed: 433
PASS: gdb.reverse/until-precsave.exp: save process recfile
elapsed: 104

Based on the performance of other tests these two test configurations
have their default timeout set to 450s and 60s respectively.

The remaining two multilibs (`-mthumb -march=armv4t' and `-mthumb
-march=armv7-a') do not produce test results usable enough to have data
available for these cases.

 Based on these results I have tweaked timeouts for these cases as
follows.  This, together with a suitable board timeout setting, removes
timeouts for these cases.  Note that for the default timeout of 10s the
new setting for the first case in gdb.reverse/until-precsave.exp is
compatible with the old one, just a bit higher to keep the convention
of longer timeouts to remain multiples of 30s.  The second case there
does not need such a high setting so I have lowered it a bit to avoid
an unnecessary delay where this test case genuinely times out.

	* gdb.reverse/sigall-reverse.exp: Increase the timeout by
	a factor of 2 for a slow test case.  Take the `gdb,timeout'
	target setting into account for this calculation.
	* gdb.reverse/until-precsave.exp: Increase the timeout by
	a factor of 15 and 3 respectively rather than adding 120
	for a pair of slow test cases.  Take the `gdb,timeout'
	target setting into account for this calculation.