[patch v2 3/5, nios2] fix register save offset for signal handler trampolines

[Sandra Loosemore]

It was requested that I split the patch previously posted here:

https://sourceware.org/ml/gdb-patches/2015-04/msg00907.html

into two parts.  This is the first part.  I also expanded the comments 
to indicate how the register save offset magic number was calculated.

OK to commit now?

-Sandra

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