Expose Cachegrind start/stop instrumentation requests

[Kobzol]

@nnethercote is now in the process of adding client requests to Cachegrind. This PR exposes these requests from crabgrind.

Cachegrind normally uses the cg_ prefix, but I noticed that in crabgrind it's already "reserved" by Callgrind (which usually uses cl_. So I used cag_ in this PR to differentiate Callgrind and Cachegrind.

[2dav]

Thank you for pointing this out, i think it's better to stick to "conventional" two-letter tool prefixes to avoid confusion, so :

• vg valgrind the app
• cl callgrind
• cg cachegrind
• hg helgrind
• mc memcheck
• ms massif
  there is now a develop branch with fixes. Could you change your PR accordingly?

[Kobzol]

Do you want me to change the base of this PR to develop or just rebase it on top of develop for now and then after it gets merged, rebased it again on top of main?

[2dav]

Change the prefix from cag_ to cg_ and the base to 'develop', i believe.

[Kobzol]

Ok, done :)

[2dav]

Thank you!
There is a missing link for Cachegrind requests doc, @Kobzol mind if i create an issue about this and assign it to you?

[Kobzol]

Sure, go ahead :)
https://valgrind.org/docs/manual/cg-manual.html it should appear here (probably once the next version of Valgrind is released).

[2dav]

available in crabgrind 0.1.10, requires Valgrind 3.22 to build.

[Kobzol]

Thanks for updating the docs!