RFR: 8344169: RISC-V: Use more meaningful frame::metadata_words where possible

[Fei Yang]

Hello, please review this RISC-V specific change which improves code readability.

Some background to help understand. We have following frame enumerations in file frame_riscv.hpp:

enum {
  link_offset           = -2,
  return_addr_offset    = -1,
  sender_sp_offset      =  0
};

The values are compatible with the platform ABI and are different from the other platforms like x64 or aarch64. Especially, `sender_sp_offset` is 0 for RISC-V compared to 2 for the other platforms. As a result, there exists some differences in places where code uses value of `sender_sp_offset` as an sp offset when fp is needed. For RISC-V, we used constant number 2 instead of `sender_sp_offset` as the sp offset. But the code will be more readable if we use `frame::metadata_words` which has the same value.

Testing on linux-riscv64:
- [x] "hotspot_loom jdk_loom" (release & fastdebug)

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

Commit messages:
 - 8344169: RISC-V: Use more meaningful frame::metadata_words where possible

Changes: https://git.openjdk.org/jdk/pull/22096/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22096&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344169
  Stats: 27 lines in 7 files changed: 5 ins; 0 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/22096.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22096/head:pull/22096

PR: https://git.openjdk.org/jdk/pull/22096