The already working watchpoint break mechanism has been extended to handle "stop" instructions, with text messages.

Explanation (also in constants-mips.h):
On MIPS Simulator breakpoints can have different codes:
- Breaks between 0 and kMaxWatchpointCode are treated as simple watchpoints, the simulator will run through them and print the registers.
- Breaks between kMaxWatchpointCode and kMaxStopCode are treated as stop() instructions (see Assembler::stop()).
- Breaks larger than kMaxStopCode are simple breaks, dropping you into the debugger.

The current values are 31 for kMaxWatchpointCode and 127 for kMaxStopCode.
From the user's point of view this works the same way as the ARM stop instruction except for the break code usage detailed above.

Ported commits: r5723 (3ba78d24)

BUG=
TEST=

Review URL: http://codereview.chromium.org//7062014

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8069 ce2b1a6d-e550-0410-aec6-3dcde31c8c00