fix fortran compiler detection on FreeBSD

On FreeBSD, passing extra options to `which` causes it to report a non-zero status:

```
$ which gfortran48 -m64
/usr/local/bin/gfortran48
$ echo $?
1
```

```
$ which gfortran48
/usr/local/bin/gfortran48
$ echo $?
0
```
c_check: set $hostarch to x86_64 instead of amd64

`uname -m` returns "amd64" on some systems.
Merge pull request #470 from nolta/patch-3

fix fortran compiler detection on FreeBSD
Merge pull request #471 from nolta/patch-4

c_check: set $hostarch to x86_64 instead of amd64
Fixed a bug of sgemm sandy bridge kernel.

Reported by Julia project. JuliaLang/julia#9084
Merge branch 'develop' of github.com:xianyi/OpenBLAS into develop
Merge branch 'develop'