Revisit libffi implementation for ARC

[kolerov]

Upstream libffi code for ARC Classic contains several issues which lead to warnings and errors. I have created a PR (libffi/libffi#844) to fix them in upstream. At the same time, patched must be pushed to Buildroot too as .patch files for libffi package. Also, I found that HS6x targets do not pass libffi DejaGNU tests related to floating point numbers.

Here is a roadmap of revisiting libffi:

• A series of fixes for ARC port libffi/libffi#844
• Push patches to Buildroot.
• Find out a root cause of failing tests for HS6x.
• Fix an implementation for HS6x.