RFR: 8312569: RISC-V: Missing intrinsics for Math.ceil, floor, rint

[Ilya Gavrilin]

Please review this changes into risc-v double rounding intrinsic.
On risc-v intrinsics for rounding doubles with mode (like Math.ceil/floor/rint) were missing. On risc-v we don`t have special instruction for such conversion, so two times conversion was used: double -> long int -> double (using fcvt.l.d, fcvt.d.l).
Also, we should provide some rounding mode to fcvt.x.x instruction. Rounding mode selection on ceil (similar for floor and rint): according to Math.ceil requirements: Returns the smallest (closest to negative infinity) value that is greater than or equal to the argument and is equal to a mathematical integer (Math.java:475). So, for double -> long int we choose rup (round towards +inf) mode to get the integer that more than or equal to the input value, for long int -> double we choose rdn (rounds towards -inf) mode to get the smallest (closest to -inf) representation of integer that we got after conversion.
For cases when we got inf, nan, or value more than 2^63 return input value (double value which more than 2^63 is guaranteed integer), as well when we store result we copy sign from input value (need for cases when for (-1.0, 0.0) ceil need to return -0.0).
We have observed significant improvement on hifive and thead boards.
testing: tier1 and tier2 on hifive

Performance results on hifive (FpRoundingBenchmark.testceil/floor/rint):

Without intrinsic:

Benchmark                      (TESTSIZE)   Mode  Cnt   Score   Error   Units
FpRoundingBenchmark.testceil         1024  thrpt   25  39.297 ± 0.037  ops/ms
FpRoundingBenchmark.testfloor        1024  thrpt   25  39.398 ± 0.018  ops/ms
FpRoundingBenchmark.testrint         1024  thrpt   25  36.388 ± 0.844  ops/ms

With intrinsic:

Benchmark                      (TESTSIZE)   Mode  Cnt   Score   Error   Units
FpRoundingBenchmark.testceil         1024  thrpt   25  80.560 ± 0.053  ops/ms
FpRoundingBenchmark.testfloor        1024  thrpt   25  80.541 ± 0.081  ops/ms
FpRoundingBenchmark.testrint         1024  thrpt   25  80.603 ± 0.071  ops/ms

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Commit messages:
 - Fix comments style
 - Add missing intrinsic for double rounding

Changes: https://git.openjdk.org/jdk/pull/14991/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14991&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312569
  Stats: 107 lines in 4 files changed: 105 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14991.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14991/head:pull/14991

PR: https://git.openjdk.org/jdk/pull/14991