RFR: Loom: RISCV stubs

[Fei Yang]

On Fri, 15 Apr 2022 13:10:58 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> RISCV64 port has been upstreamed recently. This makes compile on Linux RISCV64 platform.
>> Note current patch does not implement Loom on this platform, but merely provides the absolute basic build capabilities.
>> 
>> Additional testing:
>> Linux RISCV64 cross-compilation
>
> Good.  It looks like you got all the platform dependent code motion.  I'm not planning any more right now.

@coleenp :
Note that this merely provides the absolute basic build capabilities.
Unimplemented platforms (arm, s390, riscv, etc) are not supposed to work at the moment.
We might need something like ContinuationSupport::is_enabled() or some such to gate the implementation.

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PR: https://git.openjdk.java.net/loom/pull/150