RFR: 8364570: Remove LockingMode related code from riscv64

[Fredrik Bredberg]

On Wed, 6 Aug 2025 11:24:22 GMT, Fredrik Bredberg <fbredberg at openjdk.org> wrote:

> Since the integration of [JDK-8359437](https://bugs.openjdk.org/browse/JDK-8359437) the `LockingMode` flag can no longer be set by the user, instead it's declared as `const int LockingMode = LM_LIGHTWEIGHT;`. This means that we can now safely remove all `LockingMode` related code from all platforms.
> 
> This PR removes `LockingMode` related code from the **riscv64** platform.
> 
> When all the `LockingMode` code has been removed from all platforms, we can go on and remove it from shared (non-platform specific) files as well. And finally remove the `LockingMode` variable itself.

@RealFYang
I've run rudimentary tests using QEMU, but it would be nice if you could take it for a spin on real hardware.

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PR Comment: https://git.openjdk.org/jdk/pull/26658#issuecomment-3160395253