RFR: 8293566: RISC-V: Clean up push and pop registers

[Feilong Jiang]

Currently, riscv port has two implementations of `push_reg` and `pop_reg`.
The bitset version can be replaced with the RegSet version for better readability.

For example, if we want to push x10 and x11, pushing register with bitset would be like:

push_reg(0xc00, sp)


while pushing register with RegSet will be more clear:

push_reg(RegSet::of(x10, x11), sp);


So we decide to remove the bitset version of push and pop registers.

Testing:

- hotspot and jdk ter1 on QEMU without new failures

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Commit messages:
 - clean up push and pop registers

Changes: https://git.openjdk.org/jdk/pull/10227/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10227&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293566
  Stats: 68 lines in 4 files changed: 0 ins; 32 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/10227.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10227/head:pull/10227

PR: https://git.openjdk.org/jdk/pull/10227