Add support for SoftFloat-3e library

Currently, the arm-sflt build fails, because four {{__aeabi_*_glibc}} symbols are intended to be provided by an external library.

The intended solution adds new integration code for SoftFloat-3e library. It adds appropriate configure options and a switchable implementation of {{__aeabi_*_glibc}}/{{__aeabi_*_extlib}} functions.

There will be two implementations of the {{__aebi_*_glibc}} functions (now renamed to {{__aeabi_*_extlib}}). The simpler one just wraps the standard {{__aeabi_*}} functions. The other one wraps the SoftFloat-3 API. By using this mechanism, the distributor will have the choice. The external library is not required - if the path to its static library/include directory is not specified, the standard system functions are used automatically.

I am aware of the fact that processors older than ARMv7 are not supported in OpenJDK. However, we are so far successfully patching new OpenJDK to run on the Lego Mindstorms EV3 brick (ARM926EJ-S CPU). The project can be found at https://github.com/ev3dev-lang-java/openjdk-ev3 . We want to minimize the number of maintained patches, so we want to fix upstream whatever is possible.