RFR: 8260368: [PPC64] GC interface needs enhancement to support GCs with load barriers

[Niklas Radomski]

At present, the `needs_frame` flag is used on the ppc platform to determine whether gc barriers must emit a new stack frame (and save the link register, for that matter) or not. With the introduction of load reference barriers, however, this mechansim is no longer sufficient. This holds especially true for compiler stubs as those make heavy use of volatile registers.

To mitigate this, this patch replaces the `needs_frame` flag with a simple enumeration. As the enumerators are incremental, handling the different "register preservation needs" in the actual gc barrier implementations is comparatively (pun intended) easy. 

_This is a preparational change for the ShenandoahGC port to ppc. As such, it may provide some functionality this version doesn't make use of, but that is required for the upcoming change. This way, the scope of the upcoming change is limited to GC-specific functionality; making its review a little easier._

_For the same reason, this patch also introduces patching support for `LIR_Assembler::leal`._

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Commit messages:
 - 8260368: Enhance gc interface for advanced gc barriers

Changes: https://git.openjdk.java.net/jdk/pull/2302/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2302&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260368
  Stats: 306 lines in 19 files changed: 120 ins; 20 del; 166 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2302.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2302/head:pull/2302

PR: https://git.openjdk.java.net/jdk/pull/2302