RFR: JDK-8314056 Remove runtime platform check from frem/drem

[Scott Gibbons]

Remove platform check and move code to stubGenerator.  This fix increases performance by ~4.5%.

Tested tier1.

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Commit messages:
 - Fix register sage; compare against ah.
 - Windows changes
 - Accelerate fmod/dmod by reducing pathlength.

Changes: https://git.openjdk.org/jdk/pull/15210/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15210&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314056
  Stats: 146 lines in 6 files changed: 56 ins; 73 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/15210.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15210/head:pull/15210

PR: https://git.openjdk.org/jdk/pull/15210