RFR: 8294920: Missing SP value in Linux x86_32 thread context

[Aleksey Shipilev]

See the description in the bug. This fixes only Linux x86_32, because only there I could reproduce the failure. Windows x86_32 seems to be fine. I have no access to BSD x86_32.

Additional testing:
 - [x] Linux x86 fastdebug `serviceability/sa` now cleanly passes

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/10598/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10598&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294920
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10598.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10598/head:pull/10598

PR: https://git.openjdk.org/jdk/pull/10598