RFR: 8266813: Use shorter instruction sequence for checking if marking in progress

[Zhengyu Gu]

ShenandoahBarrierSetAssembler::generate_c1_pre_barrier_runtime_stub() on aarch64, we can replace ld, tst, br instruction sequence with a single tbz.

Test:
- [x] hotspot_gc_shenandoah on Linux aarch64

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Commit messages:
 - v1

Changes: https://git.openjdk.java.net/jdk/pull/3948/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3948&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266813
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3948.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3948/head:pull/3948

PR: https://git.openjdk.java.net/jdk/pull/3948