RFR: 8226822: GC interface for C1 runtime calls

[Roman Kennke]

 Currently, it is not possible for GC to emit runtime calls in C1
without touching shared code, because it needs to be known in
c1_Runtime.hpp/cpp.

This patch adds a little hook to allow the GC to make runtime calls known.

This is a prerequisite for:
https://bugs.openjdk.java.net/browse/JDK-8226695


Bug:
https://bugs.openjdk.java.net/browse/JDK-8226822
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8226822/webrev/

Can I please get reviews?

Thanks, Roman