RFR: JDK-8314056 Remove runtime platform check from frem/drem [v2]

[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:

  Move mxcsr code after denom > num test.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15210/files
  - new: https://git.openjdk.org/jdk/pull/15210/files/0bae5e93..5b1d251f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15210&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15210&range=00-01

  Stats: 11 lines in 1 file changed: 6 ins; 5 del; 0 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