RFR: JDK-8285675: Temporary fix for arm32 SafeFetch

[Thomas Stuefe]

[JDK-8283326](https://bugs.openjdk.java.net/browse/JDK-8283326) reworked the SafeFetch implementation and, on most platforms, switched to static assembly.

On arm (32-bit), this change led to strange crashes ([JDK-8284997](https://bugs.openjdk.java.net/browse/JDK-8284997)). These problems are time consuming to analyze (among other things they only occur when building natively on a Rasberry Pi with one particular Docker setup).

As a temporary workaround I'd like to switch SafeFetch to the sigjmp method for arm, until I find time to analyze these problems in more detail.

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Commit messages:
 - arm-safefetch-sigjmp

Changes: https://git.openjdk.java.net/jdk/pull/8399/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8399&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285675
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8399.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8399/head:pull/8399

PR: https://git.openjdk.java.net/jdk/pull/8399