Fix build error in OpenBLAS on rhel6

Use a newer assembler to allow avx2 instructions. This is the same error
that was [reported here](JuliaLang/julia#7653)
and [here](spack#12235).

```
../kernel/x86_64/dgemm_kernel_4x4_haswell.S:1709: Error: no such
instruction: vpermpd $ 0xb1,%ymm0,%ymm0'
../kernel/x86_64/dgemm_kernel_4x4_haswell.S:1709: Error: no such
instruction:vpermpd $ 0x1b,%ymm0,%ymm0'
```

var/spack/repos/builtin/packages/openblas/package.py

@@ -91,6 +91,9 @@ class Openblas(MakefilePackage):
     conflicts('%intel@16', when='@0.2.15:0.2.19')
     conflicts('~shared~static')
 
+    # Use newer assembler to allow avx2 instructions
+    depends_on('binutils', type='build', when='os=rhel6')
+
     @property
     def parallel(self):
         # unclear whether setting `-j N` externally was supported before 0.3