Use more obvious name of BGQ compiler MPI wrappers

While it is true that mpicc points to the XL compilers when it is the
one found in xl.ndebug/bin, this is not generally true and thus the use
of mpicc as a wrapper for XL violates the principle of least surprise.
the mpixl* wrappers are unambiguous and equally valid.

Fixes #1282

Change-Id: I460e829e55a221d8be35a70932864555feb21a95
Define both C and C++ compilers in BGQ toolchains; use -O3

When using either the C or C++ toolchain, the complimentary compiler is
not properly identified, rather /usr/bin/g{cc,++} is, which may cause
configuration problems.  this patch fixes this issue.

ALCF strongly recommends the use of -O3 unless there is clear evidence
that higher optimization has a (positive) effect on the quality of the
code generation from XL.  we find that XL frequently generates no faster
and even slower code with -O4 and -O5 unless the code is specifically
designed to be optimized for BGQ.

Fixes #1283

Change-Id: If32a507c6394e23106555741bdefbc0d554c3765