Comment 0 for bug 1905459

Concurrently throwing exceptions in multiple threads crashes.

This bug has impacted multiple commercial projects, some examples:
https://forums.xilinx.com/t5/Embedded-Development-Tools/Re-building-the-SDK-libc-newlib-and-libgcc-for-reentrancy/m-p/1164973
https://www.arangodb.com/2019/09/when-exceptions-collide/
https://forums.freertos.org/t/freertos-and-c-exceptions/10911

BACKGROUND
Exception unwinding requires some datastructures; specifically
the libgcc module unwind-dw2-fde.c has two-three statics.
These statics are protected by a gthread_mutex.

BUG
Arm build of libgcc builds gthread_mutex as no-op.
Thus applications cannot provide for example an RTOS
mutex with priority inheritance to protect above statics.
If I understand correctly, ARM toolchain build includes:
  gthr-single.h
which dummies out the gthread functions as inline no-ops;
consequently they cannot be overridden.

SUGGESTIONS
A couple different approaches to fix this:

1) Create+use a separate gthreads module, including weak
no-op functions for the gthreads functions. Thus RTOS users
can provide an alternate implementation of gthreads
appropriate for their RTOS, and non-RTOS users are
unaffected.

2) Use thread-local storage instead of statics in unwind-dw2-fde.c?
Is there a architecture-independent TLS wrapper in gcc libcc?

Please let me know if you agree, and if I can help...
Thanks!
Best Regards, Dave