RFR: 8285630: Fix a configure error in RISC-V cross build [v2]

[Pengfei Li]

> 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.

Pengfei Li has updated the pull request incrementally with one additional commit since the last revision:

  Address comments

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8420/files
  - new: https://git.openjdk.java.net/jdk/pull/8420/files/3d03e6d7..39031f5f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8420&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8420&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8420.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8420/head:pull/8420

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