RFR: 8339128: Cannot resolve user specified tool properly after JDK-8338304 [v2]

[Jiangli Zhou]

> Please review this PR that fixes JDK build configure failure when using clang after JDK-8338304:
> 
> 
> Found candidate GCC installation: <snip>
> Selected GCC installation: <snip>
> ...
> clang: error: unsupported option '-V -static-libgcc'
> configure:85215: $? = 1
> configure:85204: <snip> -qversion >&5
> clang: error: unknown argument '-qversion'; did you mean '--version'?
> ...
> 
> 
> Tool from Clang GCC installation is being picked up unexpectedly (JDK build configured with `--with-toolchain-type=clang`). With this fix, it avoids `UTIL_REQUIRE_PROGS(LLD, lld)` if user supplies compiler toolchain. The user supplied compiler toolchain is used by setting `CXX=<user_specified_path_to_tools_script>` during configuration.

Jiangli Zhou has updated the pull request incrementally with one additional commit since the last revision:

  Update to set CC_IS_USER_SUPPLIED and CXX_IS_USER_SUPPLIED. Only do UTIL_REQUIRE_PROGS(LLD, lld)
  if both CC_IS_USER_SUPPLIED and CXX_IS_USER_SUPPLIED are false.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21817/files
  - new: https://git.openjdk.org/jdk/pull/21817/files/9558607c..e35542e6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21817&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21817&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21817.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21817/head:pull/21817

PR: https://git.openjdk.org/jdk/pull/21817