[build] Rename x86 project to "x64"

Build output will now be in build-x64 instead of build-x86
Renamed and updated various scripts as appropriate

Within zircon, $ARCH remains "x86".
This avoids having to rename kernel/arch/x86/ and change a lot of #include paths
Removed unnecessary $SUBARCH build variable

Renamed the kernel target from pc-x86 to pc, and the board name in boards.list
from "x86" to "pc".

Change-Id: I0eb44a21ba731653b92f3ce2b78a7819c0a8f61c