syscall: fix system calls with misaligned arguments on freebsd/arm
syscall: fix system calls with misaligned arguments on freebsd/arm

This CL enables the current tree to work with FreeBSD 10-STABLE
on ARM EABI platforms, though there are still a few test fails.

Also updates documentation.

LGTM=iant
R=iant, dave
CC=golang-codereviews
https://codereview.appspot.com/61060044

pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme