[Fuchsia] Do not enable the Z3 solver for a fuchsia toolchain

gLinux started shipping incompatible versions of Z3, which can lead to a
missing `z3.h` header when building the Z3 solver locally. This patch
disables the Z3 solver when building a clang toolchain for Fuchsia.

Differential Revision: https://reviews.llvm.org/D79974