[Solaris] get rid of _RESTRICT_KYWD warning during the build

Summary:
(re)definition of _RESTRICT_KYWD rightfully causes a warning message during the Solaris build.
This hack is not needed if build compiler is properly configured (.e.g /usr/bin/gcc) so just remove it.

Reviewers: ro, mgorny, krytarowski, joerg

Reviewed By: joerg

Subscribers: quenelle, llvm-commits

Patch by Fedor Sergeev (Oracle).

Differential Revision: https://reviews.llvm.org/D35054

llvm-svn: 307469