RFR: 8256051: nmethod_entry_barrier stub miscalculates xmm spill size on x86_32

[Zhengyu Gu]

nmethod_entry_barrier stub miscalculates xmm spill size on x86_32, instead of 4 * wordSize, it uses 2 * wordSize.

This bug only affects Shenandoah, as it is the only GC that supports concurrent class unloading on x86_32.

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Commit messages:
 - Fix xmm spill size on x86_32

Changes: https://git.openjdk.java.net/jdk/pull/1170/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1170&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256051
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1170.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1170/head:pull/1170

PR: https://git.openjdk.java.net/jdk/pull/1170