[Arm] Stack pointer setup thumb

Hello Nick

I hope you are have some memories on this fix :)
https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=commit;h=026f9d290f5fe85fe9bd3a56be378fc794e28473

Is this still relevant ?

The problem is that users complaining that it has different behavior
in ARM and Thumb modes for one Cortex-R core. And they do expected
that library will make some minimal initialization in both ISA
versions.

Should we just remove this ?
+#ifdef __thumb2__
+       /* XXX Fill in stack assignments for interrupt modes.  */
+#else

Alex