RFR: 8345179: RISC-V: Add gtests for weak cmpxchg

[Robbin Ehn]

Hi, please consider.

This adds tests of aligned weak narrow cmpxchg.

[ RUN      ] RiscV.cmpxchg_weak_int16_lr_sc_vm
[       OK ] RiscV.cmpxchg_weak_int16_lr_sc_vm (2 ms)
[ RUN      ] RiscV.cmpxchg_weak_int8_lr_sc_vm
[       OK ] RiscV.cmpxchg_weak_int8_lr_sc_vm (0 ms)
[ RUN      ] RiscV.cmpxchg_weak_int16_maybe_zacas_vm
[       OK ] RiscV.cmpxchg_weak_int16_maybe_zacas_vm (0 ms)
[ RUN      ] RiscV.cmpxchg_weak_int8_maybe_zacas_vm
[       OK ] RiscV.cmpxchg_weak_int8_maybe_zacas_vm (0 ms)
[----------] 4 tests from RiscV (20997 ms total)

Executed with -XX:+UnlockExperimentalVMOptions -XX:+UseZacas

Thanks, Robbin

-------------

Commit messages:
 - gtest weak narrow cmpxchg

Changes: https://git.openjdk.org/jdk/pull/22476/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22476&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345179
  Stats: 74 lines in 1 file changed: 74 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22476.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22476/head:pull/22476

PR: https://git.openjdk.org/jdk/pull/22476