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.

Fix modf() (I hope. Can't be any worse than the old version.)

Fix stack BIAS issues.

Add sparc64-specific libc.