Introduce the PROC_SIGCODE() macro

[kd]

Summary

Use a getter macro instead of fetching the sigcode address directly from a sysent of a given process.
It assumes that the sigcode is stored in the shared page, which is true in all cases, except for a.out binaries.
This will be later useful when the shared page address randomization is introduced.
No functional change intended.

[emaste]

@mindal_semihalf.com do you have these changes in a git repo somewhere? I can merge them as they're reviewed/approved.

[kd]

In D35392#803339, @emaste wrote:

@mindal_semihalf.com do you have these changes in a git repo somewhere? I can merge them as they're reviewed/approved.

I'd prefer to do a bit more testing first.
I'll push everything to a public github repo in a day, or two.