RFR: 8354471: Assertion failure with -XX:-EnableX86ECoreOpts

[Volodymyr Paprotski]

The check to choose between AVX2 and AVX512 implementation was relying on `EnableX86ECoreOpts`. It should be relying on `supports_avxifma` and mirror the `UseIntPolyIntrinsics` check in `vm_version_x86.cpp`.

Note, in `stubGenerator_x86_64.cpp`, entry to the patched function is protected already:

  if (UseIntPolyIntrinsics) {
    StubRoutines::_intpoly_montgomeryMult_P256 = generate_intpoly_montgomeryMult_P256();
    StubRoutines::_intpoly_assign = generate_intpoly_assign();
  }

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

Commit messages:
 - Fix ECore check to be same as UseIntPolyIntrinsics

Changes: https://git.openjdk.org/jdk/pull/24644/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24644&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354471
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24644.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24644/head:pull/24644

PR: https://git.openjdk.org/jdk/pull/24644