Add "soft breakpoint" support for ARM.

Add "soft breakpoint" support for ARM.

There is some broken hardware like the HTC Desire HD where we can't
use any signals for single-stepping and breakpoints.

See http://baulig.blogspot.com/2011/04/limiting-page-faults-on-htc-desire-hd.html
for details.

To work around this, we read from a variable instead us using trigger
pages.