RFR: Make compile on other platforms.

[Coleen Phillimore]

This patch with additions and deletions of dummy stubs gets loom to compile on zero, ppc, s390, arm, linux 32 bit open, ie platforms that Oracle doesn't support.
There are two not nice 
#if !defined(AMD64) || !defined(AARCH64) || defined(ZERO)
changes.  One for platform dependent code in signature.cpp and the other is because linux zero product inlining detects the dummy null return as input to memcpy.
Also has a stack walking change that makes zero not complete compilation with the built compiler because of dummy returns.  Removing this doesn't cause any test failures in the loom tier1-3 code.

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Commit messages:
 - Make compile on other platforms.

Changes: https://git.openjdk.java.net/loom/pull/111/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=111&range=00
  Stats: 90 lines in 14 files changed: 7 ins; 55 del; 28 mod
  Patch: https://git.openjdk.java.net/loom/pull/111.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/111/head:pull/111

PR: https://git.openjdk.java.net/loom/pull/111