ispc detects avx1-i32x8 on AVX2 processor inside VM

VirtualBox environment with Ubuntu 14.04LTS

CPU:

```
$ uname -a
Linux Ubuntu-VB 3.16.0-30-generic #40~14.04.1-Ubuntu SMP Thu Jan 15 17:43:14 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
$ cat /proc/cpuinfo |grep avx2|head -n1
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx rdtscp lm constant_tsc rep_good nopl xtopology nonstop_tsc pni pclmulqdq ssse3 cx16 sse4_1 sse4_2 movbe popcnt aes xsave avx rdrand hypervisor lahf_lm abm avx2
$ cat /proc/cpuinfo |grep Intel|head -n2
vendor_id : GenuineIntel
model name : Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz
```

`ispc` output inside VM:

```
$ ispc --version
Intel(r) SPMD Program Compiler (ispc), 1.8.3dev (build commit 2c4c79d55e98753c @ 20150726, LLVM 3.5)
$ ispc
Warning: No output file or header file name specified. Program will be compiled and warnings/errors will be issued, but no output will be generated.
Warning: No --target specified on command-line. Using default system target "avx1-i32x8".
```

On OSX:

```
$ ./ispc --version
Intel(r) SPMD Program Compiler (ispc), 1.8.3dev (build commit ea89cb65bb8e5e8f @ 20150720, LLVM 3.5)
$ ./ispc
Warning: No output file or header file name specified. Program will be compiled and warnings/errors will be
issued, but no output will be generated.
Warning: No --target specified on command-line. Using default system target "avx2-i32x8".
```