RFR: 8365166: ARM32: missing os::fetch_bcp_from_context implementation [v2]

[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.

Boris Ulasevich has updated the pull request incrementally with one additional commit since the last revision:

  Update src/hotspot/os_cpu/linux_arm/os_linux_arm.cpp
  
  Co-authored-by: Aleksey Shipilëv <shipilev at amazon.de>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26699/files
  - new: https://git.openjdk.org/jdk/pull/26699/files/ba5a37d8..b7fd42b3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26699&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26699&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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