[jdk11u-dev] RFR: 8250588: Shenandoah: LRB needs to save/restore fp registers for runtime call

[Aleksey Shipilev]

Unclean backport, because Shenandoah LRB-native is not present in 11u (was added later to support concurrent weak roots).

Additional testing:
 - [x] Linux x86_64 fastdebug `hotspot_gc_shenandoah`

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Commit messages:
 - Backport 3a8a15b5d005d30ba4a7109a8e91e3e6b72b885c

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/105/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=105&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8250588
  Stats: 44 lines in 1 file changed: 26 ins; 16 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/105.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/105/head:pull/105

PR: https://git.openjdk.java.net/jdk11u-dev/pull/105