RFR: 8292867: RISC-V: Simplify weak CAS return value handling

[Aleksey Shipilev]

On Wed, 24 Aug 2022 19:23:29 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> I see that current weak CAS implementations in `macroAssembler_riscv.cpp` return `0` on success, and then invert it in `riscv.ad` to match the Java's `boolean` result. I think we can just return the final value from `MacroAssembler`.
> 
> Additional testing:
>  - [x] Linux RISC-V fastdebug `java/lang/invoke/VarHandles/`
>  - [x] Linux RISC-V fastdebug `compiler/unsafe`

Testing is clean, many (all?) potential reviewers from RISC-V land have reviewed, so I am integrating.

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PR: https://git.openjdk.org/jdk/pull/10006