Pass PICFLAGS down to cc-as-as and use __PIC__ to decide if it is small
vs big PIC mode. Retire -DPIC and -DBIGPIC.

Move UCB-licensed code from 4-clause to 3-clause licence.

Patches provided by Joel Baker in PR 22280, verified by myself.

Make __dtoul() actually DTRT. The calling convention was totally wrong, and
the `big' value should be 2^63, not 2^31.
Also, nuke __fixunsdfsi(), since this is never output by the sparc64 compiler.

Add _dtoul() which gcc generates a lot. Maybe it should go in another file.

Add sparc64-specific libc.