mpl: split libmpl into libmpl_base.la and libmpl_gpu.la

[hzhou]

Pull Request Description

MPL is used in both MPICH and hydra, as well as in standalone build of romio and libpmi. It is unnecessary and troublesome for hydra etc to acquire dependency on GPU stack. Splitting the MPL convenience library allows for finer-grain dependency.

Fixes #6986

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[hzhou]

Turns out hydra will need mpl_gpu to enable GPU binding support.