Rename sparc-myriad-elf triplet to sparc-myriad-rtems

Summary: This is to be consistent with latest Movidius MDK releases.
Also, don't inherit any gcc paths for shave triple.

Reviewers: jyknight

Subscribers: emaste, fedor.sergeev

Differential Revision: https://reviews.llvm.org/D41295

llvm-svn: 321080