RFR: 8286301: Port JEP 425 to RISC-V [v4]

[Fei Yang]

> Hi,
> 
> Please review this PR porting JEP 425 (Virtual Threads) to RISC-V.
> 
> This is mainly adapted from the work of AArch64 port. Most of the changes lie in RISC-V scope.
> Changes to HotSpot shared code are trivial and are always guarded by RISCV64 macro. So this won't
> affect the rest of the world in theory.
> 
> There exists some differences in frame structure between AArch64 and RISC-V.
> For AArch64, we have:
> 
> enum {
>   link_offset           = 0,
>   return_addr_offset    = 1,
>   sender_sp_offset      = 2
> };
> 
> While for RISC-V, we have:
> 
> enum {
>   link_offset           = -2,
>   return_addr_offset    = -1,
>   sender_sp_offset      =  0
> };
> 
> So we need adapations in some places where the code relies on value of sender_sp_offset to work.
> Note that implementation for Post-call NOPs optimization is not incorporated in this PR as we plan to
> evaluate more on its impact on performance.
> 
> Testing on Linux-riscv64 HiFive Unmatched board:
>   - Minimal, Client and Server release & fastdebug build OK.
>   - Passed tier1-tier4 tests (release build).
>   - Passed jtreg tests under test/jdk/java/lang/Thread/virtual with extra JVM options: -XX:+VerifyContinuations -XX:+VerifyStack (fastdebug build).
>   - Performed benchmark tests like Dacapo, SPECjvm2008, SPECjbb2015, etc. to make sure no performance regression are introduced (release build).

Fei Yang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Code cleanup
 - Merge branch 'master' into 8286301
 - Merge branch 'master' into 8286301
 - Fix
 - 8286301: JEP 425 to RISC-V

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10917/files
  - new: https://git.openjdk.org/jdk/pull/10917/files/06302f9d..850c8958

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10917&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10917&range=02-03

  Stats: 3921 lines in 192 files changed: 2334 ins; 976 del; 611 mod
  Patch: https://git.openjdk.org/jdk/pull/10917.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10917/head:pull/10917

PR: https://git.openjdk.org/jdk/pull/10917