RFR: 8319429: Don't zero out mxcsr flag bits on ECore

[Volodymyr Paprotski]

Improves vector rounding on ECore about 10x

(BEFORE) FpRoundingBenchmark.test_round_float        2048  thrpt    3  40.912 ± 0.044  ops/ms
(AFTER ) FpRoundingBenchmark.test_round_float        2048  thrpt    3  431.682 ± 0.727  ops/ms

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Commit messages:
 - Don't zero out mxcsr flag bits on ECore

Changes: https://git.openjdk.org/jdk/pull/16504/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16504&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319429
  Stats: 18 lines in 5 files changed: 9 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/16504.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16504/head:pull/16504

PR: https://git.openjdk.org/jdk/pull/16504