RFR: 8291302: ARM32: nmethod entry barriers support [v2]

[Aleksei Voitylov]

> This PR implements nmethod entry barriers for ARM32. It has already been implemented for other ports and is related with JDK-8290025 "Remove the Sweeper".

Aleksei Voitylov has updated the pull request incrementally with one additional commit since the last revision:

  save float registers

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11442/files
  - new: https://git.openjdk.org/jdk/pull/11442/files/828c39d0..34a17994

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11442&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11442&range=00-01

  Stats: 10 lines in 2 files changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11442.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11442/head:pull/11442

PR: https://git.openjdk.org/jdk/pull/11442