[Driver] Move information about toolchain specific include directories

from the common driver code to the corresponding `MultilibSet` declarations.

Now the `MultilibSet` can hold an optional callback function which is
responsible to return a set of include directories specific for the toolchain.
That allows to remove MIPS toolchain specific directories from
`Linux::AddClangSystemIncludeArgs` method and simplify adding new directories
in the future.

llvm-svn: 214949