[clang] Add gcc-toolset-10 support (RHEL/CentOS 8)

Clang only adds GCC paths for RHEL <= 7 'devtoolset-<N>' Software
Collections (SCL).  This generalizes this support to also include the
'gcc-toolset-10' SCL in RHEL/CentOS 8.

Reviewed By: stephan.dollberg

Differential Revision: https://reviews.llvm.org/D108908