[riscv-port] RFR: 8276989: riscv: C1 should not allocate x3, which is the gp register

[Fei Yang]

On Mon, 15 Nov 2021 03:03:41 GMT, zhengxiaolinX <duke at openjdk.java.net> wrote:

> > Changes looks good to me. Thanks for fixing this. Did you performed any tests for this?
> 
> Yes - hotspot/jdk full tiers' results look good. This patch is directly cherry-picked from [that patch](https://github.com/riscv-collab/riscv-openjdk/pull/2). Would you like to have another simple review? Thanks.

OK. Thanks. I think we still need another Reviewer here.  @shipilev

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PR: https://git.openjdk.java.net/riscv-port/pull/3