RFR: 8302460: System C++ Compiler for cross compiling gcc is dangerously wrong

[Julian Waters]

Full details in linked entry, man did this give me a scare on my local branch

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

Commit messages:
 - toolchain.m4

Changes: https://git.openjdk.org/jdk/pull/12558/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12558&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302460
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12558.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12558/head:pull/12558

PR: https://git.openjdk.org/jdk/pull/12558