As x86-64 is a superset of x86, we can use qemu-system-x86_64 to run
the OS image.
Signed-off-by: Antonio Gutierrez <chibby0ne@gmail.com>

switchuvm() is supposed to switch the TSS and page table to the
process p it is passed. Alas, instead of using p to access the
kstack field, it used the global proc. This worked fine because
(a) most uses of switchuvm() pass proc anyway and (b) because in
the schedule, where we call switchuvm with the newly scheduled
process, we actually set the global proc before the call. But I
think it's still a bug, even if it never broke a test case. :-)

Since readi() returns -1 for errors, checking with < against an unsigned
value is inadvisable. Checking with != works as intended however.

when a zero PDE is encountered while searching for present PTEs to free,
resume searching at first entry of the next page table instead of the
current entry of the next page table.

On platforms with NaCl support, ld.gold -V will return a "elf_i386_nacl" in
addition to "elf_i386", which will make the build fail.

fix bugs where caller incorrectly released lock on error return path.

as a direct index into cpus.  Record apicid in struct cpu and have cpunum() look
for it. Replace cpu->id with cpunum() everywhere, and replace cpu->id with cpu->apicid.
Thanks to Xi Wang.

add to test that they indeed trap in user space
thanks to pablogventura@gmail.com and nwolovick@gmail.com