RFR: JDK-8314056 Remove runtime platform check from frem/drem [v3]

[Scott Gibbons]

> Remove platform check and move code to stubGenerator.  This fix increases performance by ~4.5%.
> 
> Tested tier1.

Scott Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  mxcsr fix; address review comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15210/files
  - new: https://git.openjdk.org/jdk/pull/15210/files/5b1d251f..31e6000e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15210&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15210&range=01-02

  Stats: 51 lines in 4 files changed: 11 ins; 14 del; 26 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