SIGSEGV due to wrong stolen reg value after synchall

While working on PR #4491 which adds a synchall to drcachesim while enabling tracing, I encountered a crash due to the stolen reg having a value that doesn't seem like an address.

The segfaulting threads were redirected to the reset exit stub by DR at [1]. The SIGSEGV occurs in the exit stub shown below.

Thread xxx received signal SIGSEGV, Segmentation fault.
0x0000000046fc2458 in ?? ()
(gdb) x/50i 0x46fc2458
=> 0x46fc2458:	stp	x0, x1, [x28]
   0x46fc245c:	mov	x0, #0x4c20                	// #19488
   0x46fc2460:	movk	x0, #0xf7f0, lsl #16
   0x46fc2464:	movk	x0, #0xffff, lsl #32
   0x46fc2468:	ldr	x1, [x28, #64]
   0x46fc246c:	br	x1
   ...

The exit stub expects the stolen reg to be set up already, but it doesn't seem to be -- see x28 below.

(gdb) i r 
x0             0xfffffffffffffffc  -4
x1             0xfffff45bcc80      281474781400192
...
x28            0x30000             196608
...
pc             0x46fc2458          0x46fc2458

The issue was fixed by adding a stolen reg store. Also there's a bug in the reset tests linux.thread-reset and linux.clone-reset, due to which they don't fail on AArch64 when they should.

Filing and fixing these separately from the PR #4491.

[1]: translate_from_synchall_to_dispatch https://github.com/DynamoRIO/dynamorio/blob/9bedb9c72d5808785e28d20df3ed1c932b555ac9/core/synch.c#L1816