Wrap ARM abort() to improve stack trace.

Wrap ARM abort() to improve stack trace.

The code generated for Thumb and Thumb2 targets has different handling
for abort().  Because abort() is "noreturn", it doesn't need to preserve
the callee-save registers.  The Thumb2 version trashes LR and makes it
impossible to figure out who called abort().

This inserts a trivial stub function; net effect is stack traces are
reasonable after an abort().

For bug 2191452.

Eclair branch Dr. No approved by: hiroshi