scons: Increase the minimum clang version to 3.9.

scons: Increase the minimum clang version to 3.9.

This matches what's documented elsewhere. We *need* version 3.4 to
support c++14, but we support only as far back as 3.9. Also, the
argument to set c++14 as the standard is different in 3.4 and earlier
(-std=c++1y), so it makes life slightly easier to move past it to 3.9.

Change-Id: I66fa578dd3222c62907496a888f8068ed0918c7b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34819
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>