RFR(M): 8215902: Add support for SoftFloat-3e library

[Jakub Vaněk]

Hi,

please review this webrev. It is a successor of the softfloat-3 [patch]
thread (first email 
http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2018-November/031311.html
)

Changes since the last patch (v6):

- renamed --with-softloat* to --with-sflt* (it is more compact and it
  corresponds to the old --with-sflt-lib=... option)

- license is now obtained via --with-sflt-license switch (so it is not
  included in OpenJDK source tree)

- updated documentation (slight rewording, added the license option)

- checks for default --with/--without behavior are in place again
  (I forgot them when I changed the way the library is detected)

- added a simple testcase - I found a disrepancy between softfloat and
  system function behavior. When a float with bits 0x003FFFFF is
  added to 0x00000001, the correct result is 0x00400000, but the
  default software floating point implementation returns 0x00000000.
  However I'm not sure where to put this test - now it is in
  test/hotspot/jtreg/compiler/floatingpoint.

- comments in code refer to CR 6757269 and newly JDK-8215902 too.

I have created a repository with SoftFloat-3e with build configuration
specifically for OpenJDK on armel:
https://github.com/ev3dev-lang-java/softfloat-openjdk

I can add a link to it to the documentation.

Bug: https://bugs.openjdk.java.net/browse/JDK-8215902
Webrev: http://cr.openjdk.java.net/~jakvanek/8215902/webrev.02/
CI build: https://ci.adoptopenjdk.net/view/ev3dev/job/openjdk12_build_ev3_linux/67/

Cheers,

Jakub