[Solaris] Make RHEL devtoolsets handling Linux-specific

Summary:
This patch is meant to address the last outstanding review comment on the already approved
(but not yet commited) https://reviews.llvm.org/D35755, namely making the handling of the RHEL
devtoolsets Linux-specific.

Don't know if it's best integrated into the former or applied subsequently.

Tested on i386-pc-solaris2.11 and x86_64-pc-linux-gnu.

Reviewers: fedor.sergeev, tstellar, jyknight

Reviewed By: fedor.sergeev

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42029

llvm-svn: 323194