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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