RFR: 8365166: ARM32: missing os::fetch_bcp_from_context implementation

[Boris Ulasevich]

Since [JDK-8352251](https://bugs.openjdk.org/browse/JDK-8352251) (JEP 518: JFR Cooperative Sampling), jfrSampleRequest calls os::fetch_bcp_from_context(). On ARM32 port it is still Unimplemented(). It should read the corresponding field from uc_mcontext and return the BCP value.

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Commit messages:
 - ensure Rbcp is R7
 - 8365166: ARM32: missing os::fetch_bcp_from_context implementation

Changes: https://git.openjdk.org/jdk/pull/26699/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26699&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365166
  Stats: 10 lines in 1 file changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26699.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26699/head:pull/26699

PR: https://git.openjdk.org/jdk/pull/26699