8387078: RISC-V: x27 can be allocated in CompressedOops mode

[zzthread]

Hi, we can see that the x27 register is used as the heapbase. If the heap is smaller than 32 GB, CompressedOops::base() is always 0, so in this case, we can provide an additional available register for C2.

Testing(SpacemiT Key Stone K3)

• tier1-3 tests (release)

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• JDK-8387078: RISC-V: x27 can be allocated in CompressedOops mode (Enhancement - P4)

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[zzthread]

/covered

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[sendaoYan]

Suggested change

	if (UseCompressedOops && (CompressedOops::base() != nullptr)) {
	if (UseCompressedOops && CompressedOops::base() != nullptr) {