[CMake][Fuchsia] Enable x86 relaxation by default

This enables x86 relaxation by default. This depends on a linker new
enough to support the new reloc types but since we default to lld we
don't worry about host system linkers that might be too old to support
the new reloc types.

Differential Revision: https://reviews.llvm.org/D56349

llvm-svn: 350460