CRASH(drmemtrace): mcontext register holding incorrect value during signal handler

**Describe the bug**

When running `drmemtrace`, the value of registers in the `mcontext` during signal handler is incorrect.

**To Reproduce**

The repository https://github.com/WilliamQiufeng/dynamorio-drmemtrace-reg-mre shows the (hopefully minimal) assembly needed to produce this difference.

* Install a signal handler for SIGSEGV
* write some magic value to `x0`
* Load an invalid address to trigger SIGSEGV
* Compare mcontext register[0]

Please also answer these questions:
- Without any client, it runs fine and the value checks out
- `-debug` option doesn't change the result. When using `drmemtrace` the values do not match.

**Expected behavior**

the `x0` saved in `mcontext` should equal the correct value.

**Versions**
- DynamoRIO 11.90.20672
- Ubuntu, Aarch64, Neoverse N1

Start with the linked minimal reproduction and compare its SIGSEGV mcontext behavior with and without drmemtrace on Ubuntu AArch64. Trace drmemtrace's signal-handler and mcontext handling to determine why register[0] differs. Done means the saved x0 value matches the value written before the fault when drmemtrace is enabled.

Written by the indexing model from the issue text.