RFR: JDK-8301392: Port fdlibm log1p to Java

[Joe Darcy]

Another day, another PR to port FDLBIM to Java, this time for the log1p method.

Other than using the two-argument form of the __HI method in Java transliteration version rather than C macro, there are no appreciable differences between the original C source in 

src/java.base/share/native/libfdlibm/s_log1p.c

and the transliteration for testing purposes in

test/jdk/java/lang/StrictMath/FdlibmTranslit.java

The more idiomatic port in

src/java.base/share/classes/java/lang/FdLibm.java

has had a series of transformation applied layering on the transliteration. The intermediate commits show the progress.

The regression tests include probing around input values the implementation uses to decided which branch to take.

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Commit messages:
 - Refactor to use fp hex literals, a little more whitespace refactoring.
 - More whitespace improvements.
 - Improve whitespace usage.
 - JDK-8301392: Port fdlibm log1p to Java

Changes: https://git.openjdk.org/jdk/pull/12301/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12301&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301392
  Stats: 389 lines in 4 files changed: 377 ins; 1 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/12301.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12301/head:pull/12301

PR: https://git.openjdk.org/jdk/pull/12301