x86inc: Utilize the shadow space on 64-bit Windows

Store XMM6 and XMM7 in the shadow space in functions that clobbers them.
This way we don't have to adjust the stack pointer as often,
reducing the number of instructions as well as code size.
x86: 32-byte align the stack if possible

Avoids the need for manual 32 byte array alignment on compilers that support
-mpreferred-stack-boundary.
x86-64: faster SSSE3 trellis

~2% faster trellis.
x86: faster SSSE3 hpel

~7% faster using the pmulhrsw trick from mc_chroma.
x86: Faster high bit-depth intra_sad_x3_4x4

20->16 cycles on Ivy Bridge
x86: AVX2 deblock strength

30->18 cycles
x86: AVX2 high bit-depth intra_sad_x3_8x8

43->24 cycles
x86: AVX2 intra_sad_x3_8x8c

30->22 cycles
x86: faster AVX2 quant_4x4x4

10->9 cycles
x86: AVX2 add16x16_idct_dc

27 -> 19 cycles
x86: AVX2 high bit-depth quant

quant_4x4: 13->6 cycles
quant_4x4_dc: 14->8 cycles
quant_8x8: 47->24 cycles
quant_4x4x4: 48->25 cycles
x86: AVX2 high bit-depth denoise_dct

28->15 cycles

Also reorder instructions to use fewer registers, 3 cycles faster on Ivy Bridge with 64-bit Windows.
x86-64: 64-bit variant of AVX2 hpel_filter

~5% faster than 32-bit.
x86: AVX2 high bit-depth dequant
x86: AVX2 dequant_4x4_dc
x86: shave a few instructions off AVX deblock
OpenCL support improvement/refactoring

Autoload the OpenCL library so that it's not required to run an openCL-enabled
build of x264.

Update X264_BUILD, which should have been changed with the first patch.