kernel/x86_64: save fpu state at interrupts

The kernel is allowed to use fpu anywhere so we must make sure that
user state is not clobbered by saving fpu state at interrupt entry.
There is no need to do that in case of system calls since all fpu
data registers are caller saved.

We do not need, though, to save the whole fpu state at task swich
(again, thanks to calling convention). Only status and control
registers are preserved. This patch actually adds xmm0-15 register
to clobber list of task swich code, but the only reason of that is
to make sure that nothing bad happens inside the function that
executes that task swich. Inspection of the generated code shows
that no xmm registers are actually saved.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
libroot/x86_64: new memcpy implementation

This patch introduces new memcpy() implementation that improves the
performance when the buffer is small. It was written for processors that
support ERMSB, but performs reasonably well on older CPUs as well.

The following benchmarks were done on Haswell i7 running Debian Jessie
with Linux 3.16.1. In each iteration 64MB buffer was copied, the
parameter "size" is the size of the buffer passed in a single call (i.e.
for "size: 2" memcpy() was called ~32 million times to copy the whole
64MB).

f - original implementation, g - new implementation, all buffers 16 byte
aligned

cpy, size:        8, f:    79971 µs, g:    20419 µs, ∆:   74.47%
cpy, size:       32, f:    42068 µs, g:    12159 µs, ∆:   71.10%
cpy, size:      128, f:    13408 µs, g:    10359 µs, ∆:   22.74%
cpy, size:      512, f:    10634 µs, g:    10433 µs, ∆:    1.89%
cpy, size:     1024, f:    10474 µs, g:    10536 µs, ∆:   -0.59%
cpy, size:     4096, f:     9419 µs, g:     8630 µs, ∆:    8.38%

f - glibc 2.19 implementation, g - new implementation, all buffers 16 byte
aligned

cpy, size:        8, f:    26299 µs, g:    20919 µs, ∆:   20.46%
cpy, size:       32, f:    11146 µs, g:    12159 µs, ∆:   -9.09%
cpy, size:      128, f:    10778 µs, g:    10354 µs, ∆:    3.93%
cpy, size:      512, f:    12291 µs, g:    10426 µs, ∆:   15.17%
cpy, size:     1024, f:    13923 µs, g:    10571 µs, ∆:   24.08%
cpy, size:     4096, f:    11770 µs, g:     8671 µs, ∆:   26.33%

f - glibc 2.19 implementation, g - new implementation, all buffers unaligned

cpy, size:       16, f:    13376 µs, g:    13009 µs, ∆:    2.74%
cpy, size:       32, f:    11130 µs, g:    12171 µs, ∆:   -9.35%
cpy, size:       64, f:    11017 µs, g:    11231 µs, ∆:   -1.94%
cpy, size:      128, f:    10884 µs, g:    10407 µs, ∆:    4.38%
cpy, size:      256, f:    10826 µs, g:    10106 µs, ∆:    6.65%
cpy, size:      512, f:    12354 µs, g:    10396 µs, ∆:   15.85%

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>