[CMake] Support passing FUCHSIA_SDK as the only variable

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@335708 91177308-0d34-0410-b5e6-96231b3b80d8