RFR: JDK-8305782: Provide MacroAssembler::breakpoint on aarch64

[Thomas Stuefe]

The ability to emit debug traps was useful for me on arm, and I miss it on aarch64.

Tested manually on Linux aarch64 in gdb with various values for hint covering the whole 16-bit range set. Hint gets encoded in the instruction (gdb decodes instruction as "BRK xxx" with xxx being the hint). According to documentation the hint ends up in ESR.ELx.ISS after the trap hit, but gdb refused to display the ESR register, so I could not verify that.

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Commit messages:
 - implement breakpoint

Changes: https://git.openjdk.org/jdk/pull/13401/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13401&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305782
  Stats: 10 lines in 3 files changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13401.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13401/head:pull/13401

PR: https://git.openjdk.org/jdk/pull/13401