RFR: 8293007: riscv: optimize nmethod entry barrier

[Yadong Wang]

https://bugs.openjdk.org/browse/JDK-8290700 introduced a faster nmethod entry barrier: a out-of-line stub to the entry barriers of C2 methods, and also a concurrent-data-and instruction-patching barrier.
Porting loom and generational ZGC are both under development in progress in riscv port project, and we brought the similar modification to make things smooth. 

Tier1 passed on unmatched.

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Commit messages:
 - fix whitespace
 - 8293007: riscv: optimize nmethod entry barrier

Changes: https://git.openjdk.org/jdk/pull/10056/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10056&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293007
  Stats: 282 lines in 16 files changed: 210 ins; 27 del; 45 mod
  Patch: https://git.openjdk.org/jdk/pull/10056.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10056/head:pull/10056

PR: https://git.openjdk.org/jdk/pull/10056