RFR: 8285630: Fix a configure error in RISC-V cross build

[Fei Yang]

On Wed, 27 Apr 2022 09:50:35 GMT, Pengfei Li <pli at openjdk.org> wrote:

> We are trying to cross build a RISC-V version of OpenJDK. We specified
> `--openjdk-target=riscv64-linux-gnu` after `bash configure` but got an
> error message.
> 
> 
> configure: error: /usr/bin/bash /home/ent-user/jdk_src/make/autoconf/build-aux/config.sub riscv64-linux-gnu failed
> configure exiting with result code 1
> 
> 
> It shows the processing of `riscv64-linux-gnu` in script `config.sub`
> fails. We see `config.sub` calls another script `autoconf-config.sub`
> to validate and canonicalize a configuration type. The validation fails
> here because `autoconf-config.sub` is a quite old version and has no
> RISC-V support inside. Comments in those scripts tell us patching the
> autoconf script is not a good idea so we add a fix in `config.sub` in
> this patch.
> 
> We have verified RISC-V cross build succeeds after this change. As we
> are not quite familiar with the build system, please let us know if this
> is the best way to fix.

This also works when doing a native build. 
Do you know why this issue is not there for other cpus likes aarch64? Just curious. 
BTW: I noticed there is an unnecessary new line at the end of file config.sub. Maybe we should remove it.

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PR: https://git.openjdk.java.net/jdk/pull/8420