RFR: 8318158: RISC-V: implement roundD/roundF intrinsics

[Olga Mikhaltsova]

Please, review this Implementation of the roundD/roundF intrinsics for RISC-V platform.
As shown below the output for RISC-V instructions and Java methods differs only for NaN argument.

                                                  RISC-V                            Java
                                        (FCVT.W.S)    (FCVT.L.D)  (long round(double a)) (int round(float a))
Minimum valid input (after rounding)     −2^31         −2^63         Long.MIN_VALUE       Integer.MIN_VALUE
Maximum valid input (after rounding)      2^31 − 1      2^63 − 1     Long.MAX_VALUE       Integer.MAX_VALUE
Output for out-of-range negative input   −2^31         −2^63         Long.MIN_VALUE       Integer.MIN_VALUE
Output for −∞                            −2^31         −2^63         Long.MIN_VALUE       Integer.MIN_VALUE
Output for out-of-range positive input    2^31 − 1      2^63 - 1     Long.MAX_VALUE       Integer.MAX_VALUE
Output for +∞                             2^31 − 1      2^63 - 1     Long.MAX_VALUE       Integer.MAX_VALUE
Output for NaN                            2^31 − 1      2^63 - 1           0                      0

The benchmark shows the following performance improvement:

**Before**

Benchmark                              (TESTSIZE)   Mode  Cnt  Score   Error   Units
FpRoundingBenchmark.test_round_double        2048  thrpt   15  4.675 ± 0.259  ops/ms
FpRoundingBenchmark.test_round_float         2048  thrpt   15  4.549 ± 0.210  ops/ms


**After**

Benchmark                              (TESTSIZE)   Mode  Cnt   Score   Error   Units
FpRoundingBenchmark.test_round_double        2048  thrpt   15  10.483 ± 0.681  ops/ms
FpRoundingBenchmark.test_round_float         2048  thrpt   15  10.475 ± 0.480  ops/ms


Testing: tier1 tests successfully passed on a RISC-V HiFive board with Linux.

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Commit messages:
 - 8318158: RISC-V: implement roundD/roundF intrinsics

Changes: https://git.openjdk.org/jdk/pull/16382/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16382&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318158
  Stats: 31 lines in 3 files changed: 25 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/16382.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16382/head:pull/16382

PR: https://git.openjdk.org/jdk/pull/16382