misc: Replace enable_if<>::type with enable_if_t<>.

misc: Replace enable_if<>::type with enable_if_t<>.

This new abreviated form was added for C++14. Now that we're using that
version of the standard, we can move over to it.

Change-Id: Ia291d2b1e73e503c37593b1e1c4c1b3011abc63b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36477
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>