i#5233: Fix arm-vs-thumb signal transitions

[derekbruening]

Removes a too-early-and-thus-incorrect call to set_pc_mode_in_cpsr()
in execute_handler_from_cache()
(transfer_from_sig_handler_to_fcache_return() does this for us at the
right time).

Removes an incorrect call to dr_set_isa_mode from the cpsr in
transfer_from_sig_handler_to_fcache_return(): we want to only set the
mode from the target, not the interruption point.

Works around QEMU bugs with signals 63 and 64 by using 62 instead in
the linux.signalNNNN tests. This allows adding them to the list of
tests that work under QEMU.

Augments the linux.signalNNNN tests to vary whether the main code and
the signal handler are arm or thumb, helping to catch and test signal
transition issues.

Issue: #4719, #5233
Fixes #5233

[derekbruening]

Given that the new tests hit multiple different instances of the isa mode assert and these changes fix all of those, I'm pretty confident this is good to merge even if it doesn't fix every case hit in #5233 -- so going to merge and not wait for the filer to test. Enabling the signal tests for arm is great too. (Unfortunately they still fail under QEMU for aarch64 -- did not have time to figure out why; future work for #4719.)