No support for >256 CPUs.

The current implementation uses the single byte of CPU ID information in `EBX` from `CPUID` with `EAX=1` to play nicely with CPUs that do not support x2APIC (such as AMDs and older Intels). However, this limits the number of processors to 256. `CPUID` also supports giving x2APIC IDs using `EAX=0xb` and reading the 4 bytes in `EDX`. `cpu_amd64.s` should use this feature if the CPU supports it.

A patch to correctly use the x2APIC CPUID instruction is given below:

``` diff
diff --git a/cpu_amd64.s b/cpu_amd64.s
index b485f31..354f8f9 100644
--- a/cpu_amd64.s
+++ b/cpu_amd64.s
@@ -2,14 +2,12 @@

// func cpu() uint64
TEXT ·cpu(SB),NOSPLIT,$0-8
- MOVL $0x01, AX // version information
+ MOVL $0x0b, AX // version information
MOVL $0x00, BX // any leaf will do
MOVL $0x00, CX // any subleaf will do

// call CPUID
BYTE $0x0f
BYTE $0xa2
-
- SHRQ $24, BX // logical cpu id is put in EBX[31-24]
- MOVQ BX, ret+0(FP)
+ MOVQ DX, ret+0(FP) // logical cpu id is put in EDX
RET
```