I submitted it as a bug to the GNU ARM Embedded initially, see here for 
details: https://bugs.launchpad.net/gcc-arm-embedded/+bug/1566054

Basically, this patch allow gdb to unwind properly an extended stack 
frame, that is an exception frame with FPU state stacked. Additionally, 
because all Cortex-M variants have 2 stack pointers, the Main Stack 
Pointer (MSP) and the Process Stack Pointer (PSP), the code in the patch 
also check which stack was used prior to the exception. That way, 
backtraces work beautifully.

In my original submission, I mentioned a known issue that I didn't try 
to fix *yet*, because that would involve a lot more work, and the impact 
is relatively minor: for a given outer frame, some FPU registers may not 
be reported correctly. I hope you don't mind too much. I consider the 
current patch still useful, because at least backtraces work, and it's 
an annoyance not to be able to get them.

-- 
James-Adam Renquinha Henri, Ing. jr
Ingenieur d'application
CIMEQ INC.