RFR: JDK-8301392: Port fdlibm log1p to Java

[Raffaello Giulietti]

On Tue, 31 Jan 2023 02:52:45 GMT, Joe Darcy <darcy at openjdk.org> wrote:

> 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.

For consistency with the code for [log10](https://github.com/openjdk/jdk/pull/12242), the hex `int` literals could be separated with `_` for better readability, as in `0x3ff0_0000`.

Otherwise LGTM

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