Put missing END() markers to set ELF symbol size.

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

Don't clobber callee save registers r12 & r13 in swapcontext. They
were saved in the stack, but the subsequent swapcontext would return
to the earlier stack frame and never restore them. Fixes puff

Don't clobber callee save registers r12 & r13 in swapcontext. They
were saved in the stack, but the subsequent swapcontext would return
to the earlier stack frame and never restore them. Fixes puffs
with default compiler flags on amd64.

Thanks to Mihai Chelaru for providing access to an amd64 machine
for debugging this problem.

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Use correct stack offsets after pushing values onto it.

Make sure we reference getcontext() by its internal name.

Add ucontext glue for x86_64.