[cmake] Remove duplicate command line options from build

CMAKE_CXX_FLAGS and CMAKE_C_FLAGS are added twice to the command line.
This causes the command line options to be doubled which works until
it doesn't as not all options can be specified twice. 

For example,

clang-cl foo.c /GS- /GS- -mllvm -small-loop-cost=1 -mllvm -small-loop-cost=1
clang (LLVM option parsing): for the -small-loop-cost option: may only occur zero or one times!

llvm-svn: 329340