RFR: 8292694: x86_64 c2i/i2c adapters may use more stack space than necessary

[Dean Long]

The c2i adapter includes the return address in the alignment calculation, making %rsp aligned instead of %rsp+8.
The i2c adapter aligns "extrawords" even though %rsp is aligned later, and the incoming stack is not guaranteed to be aligned.

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Commit messages:
 - fix c2i and i2c stack alignment

Changes: https://git.openjdk.org/jdk/pull/10034/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10034&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292694
  Stats: 86 lines in 4 files changed: 51 ins; 25 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/10034.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10034/head:pull/10034

PR: https://git.openjdk.org/jdk/pull/10034