Enable SSE in i386

For libcompiler_rt makes use of SSE instructions in its floating point
routines, CR4's OSFXSR flag must be enabled or an #UD exception will
ensue.
This patch also enables the OSXMMEXCPT flag, just to be in sync with
the amd64 version of the boot code.

Save BIOS data area values in locore

Eliminates the need to muck with page tables on amd64

actually commit the code changes too (for previous commit)

remove bmk_ prefix from routines private to platform/hw

Pass multiboot info as an argument instead of global.

Implement the "not multibooted" message in assembly.

That way we can use the same code for amd64 (where we can't call C code
at that stage since we're not yet in long mode).

unify {i386,amd64}/boot.c -> x86/boot.c

less magic numbers

Clear console in assembly.

Avoids having to call C in very early bootstrap.

On second thought, just set _edata to the actual end of data

Load data from binary up to _tdata_end (which is after _edata)

lidt and ltr belong to locore.S, not intr.S

Install TSS for amd64

Makes the mpg123 unikernel work as amd64 on my laptop (i.e. real
bare metal instead of just qemu)

enable SSE

mpg123 with cpu=x86-64 now works

add initial support for amd64