[riscv-port-jdk11u:riscv-port] RFR: 8316645: RISC-V: Remove dependency on libatomic by adding cmpxchg 1b

[Robbin Ehn]

For CI builds it helps to remove this.
[8317335](https://bugs.openjdk.org/browse/JDK-8317335) is not needed as there is no gtest for atomic in 11.

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Commit messages:
 - Backport fb055e7e5300958b2a6a290aa6783e8ede929d9a

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/10/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=10&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316645
  Stats: 75 lines in 2 files changed: 69 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/10.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/10/head:pull/10

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/10