[dev.power64] cmd/9l, math/big, runtime, syscall: fixes for power64

On the machine I'm using, the hardware page size seems to be 64 kB.
Make ELF rounding and mmap quantum 64 kB to match.

Error numbers returned from kernel are positive; do not negate.

Implement stubs for math/big.

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/124300043