haskell-src-exts: set -mcpu flag for LLVM

Normally LLVM autodetects the cpu of the machine it's building on and optimizes for it. This can cause the binary to crash when executed on a cpu different from the one it was built on, e.g. if the build cpu supports AVX instructions and the host does not.

In order to produce generic code `-mcpu=i686` or `-mcpu=x86-64` should be passed to `llc`.

Signed-off-by: Nicola Squartini <tensor5@gmail.com>