[CMake] Support passing FUCHSIA_SDK as the only variable

Now that the structure of Fuchsia SDK has been formalized, we no
longer need to pass all the different CFLAGS/LDFLAGS to the CMake
build separately, we can simply set the FUCHSIA_SDK variable and
derive all the necessary variables from that one inside the cache
file.

Differential Revision: https://reviews.llvm.org/D48564

llvm-svn: 335708