scripts: set clang as REAL_CC to support LLVM by default

To support LLVM by default, set REAL_CC to clang which overrides
gcc.

Also, white list warnings which are being treated as errors
while compiling Kernel with clang compiler.

Change-Id: I5df2375cb8a2f424521fdd862d550ab340a34c98
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>