[riscv-port] RFR: 8278337: riscv: remove unnecessary ld/sd zr around calls

[kuaiwei]

It seems zr(x0) is used as stack alignment when saving/restoring registers around calls. But it's unnecessary to ld/st x0 from/to stack. We just need reserve a stack slot in case alignment is required.

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Commit messages:
 - 8278337: riscv: remove unnecessary ld/sd zr around call

Changes: https://git.openjdk.java.net/riscv-port/pull/25/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=25&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278337
  Stats: 19 lines in 2 files changed: 4 ins; 8 del; 7 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/25.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/25/head:pull/25

PR: https://git.openjdk.java.net/riscv-port/pull/25