RFR: JDK-8301392: Port fdlibm log1p to Java [v2]

[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.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Implement review feedback.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12301/files
  - new: https://git.openjdk.org/jdk/pull/12301/files/6bdcd26d..acd3693f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12301&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12301&range=00-01

  Stats: 18 lines in 2 files changed: 0 ins; 0 del; 18 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