[riscv-port] RFR: 8278337: riscv: remove unnecessary ld/sd zr around calls [v3]

[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.

kuaiwei has updated the pull request incrementally with one additional commit since the last revision:

fix code style

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Changes:
- all: https://git.openjdk.java.net/riscv-port/pull/25/files
- new: https://git.openjdk.java.net/riscv-port/pull/25/files/ddc32f4e..c64168c4

Webrevs:
- full: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=25&range=02
- incr: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=25&range=01-02

Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 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