RFR: 8319440: RISC-V: jdk can't be built with clang due to register keyword

[Fei Yang]

On Tue, 21 Nov 2023 16:20:20 GMT, ArsenyBochkarev <duke at openjdk.org> wrote:

> Hello everyone! I removed the `register` keyword from `read_csr` function, which is used to check CPU vector length. Inside QEMU with RVV I see that the behavior is the same as with `register`. Furthermore, inside GDB I see that register used for `csrr vlenb` is `a0` with and without `register` keyword. With this fix `make hotspot` with latest clang used succeeded without any warnings.

Marked as reviewed by fyang (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/16768#pullrequestreview-1744012520