Makefile: Remove obsolete -fno-builtin flag

CAF cherry-picked an obsolete LLVMLinux patch from 2014 in commit
96d47aed 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.

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