[riscv-port] RFR: 8278337: riscv: remove unnecessary ld/sd zr around calls [v2]

[kuaiwei]

On Wed, 8 Dec 2021 06:43:31 GMT, kuaiwei <duke at openjdk.java.net> wrote:

>> 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 refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR.

>
Hi Felix,  the patch passed tier1 test in qemu with fastdebug build.

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