Makefile: Remove obsolete -fno-builtin flag

CAF cherry-picked an obsolete LLVMLinux patch from 2014 in commit
a2a31f1d that introduced this
unnecessary flag. Now that the functions used by Clang's libcall
optimizations have been implemented, we can allow Clang to perform said
optimizations for a minor performance bump in certain code paths.

Change-Id: Ifa6b731c9702973be2c656d341e33a6186ed7cf2
Signed-off-by: Danny Lin <danny@kdrag0n.dev>