[CMake] Use explicit targets for building Linux runtimes

Previously we were using default logic when building Linux runtimes
in Fuchsia toolchain, but that leads to various issues due to how
the CMake logic in compiler-rt for determining the platform support
is implemented. With this change, we will use explicit target for
every provided Linux sysroot.

Differential Revision: https://reviews.llvm.org/D48563

llvm-svn: 335812