LIBXSMM 1.8.2 (#6896)

* Included LIBXSMM 1.8 into the list of available versions.

* LIBXSMM 1.8.1

* LIBXSMM 1.8.2 (release notes: https://github.com/hfp/libxsmm/releases/tag/1.8.2).

* LIBXSMM: Use join_path instead of hard-coding the separator. Install "version.txt" into the documentation directory. Removed installing "README.md" from LIBXSMM's root directory as it overrides README.md provided there. The latter uses correct relative references to the other documentation parts.

Note: Apparently, "FC=/path/to/gfortran spack install libxsmm" is currently needed for Spack since does not pick-up the Fortran compiler (but incorrectly uses the C compiler instead).

* LIBXSMM: converted Package into MakefilePackage (to address https://github.com/spack/spack/pull/6896#discussion_r160993923).

* LIBXSMM: account for changed file set in 1.8.2 onward (addresses https://github.com/spack/spack/pull/6896#pullrequestreview-88670974).

* Fixed incorrect behavior of "+header-only", which did not install the "src" folder. Addressed https://github.com/spack/spack/pull/6896#discussion_r162783180 and https://github.com/spack/spack/pull/6896#discussion_r162783221.

* Use conflicts msg argument to present a friendly error message.