Makefile: Add '-fno-builtin-bcmp' to CLANG_FLAGS

Makefile: Add '-fno-builtin-bcmp' to CLANG_FLAGS

After LLVM revision r355672 [1], all known working kernel
configurations fail to link [2]:

ld: init/do_mounts.o: in function `prepare_namespace':
do_mounts.c:(.init.text+0x5ca): undefined reference to `bcmp'
ld: do_mounts.c:(.init.text+0x5e6): undefined reference to `bcmp'
ld: init/initramfs.o: in function `do_header':
initramfs.c:(.init.text+0x6e0): undefined reference to `bcmp'
ld: initramfs.c:(.init.text+0x6f8): undefined reference to `bcmp'
ld: arch/x86/kernel/setup.o: in function `setup_arch':
setup.c:(.init.text+0x21d): undefined reference to `bcmp'

Commit 6edfba1b33c7 ("[PATCH] x86_64: Don't define string functions to
builtin") removed '-ffreestanding' globally and the kernel doesn't
provide a bcmp definition so the linker cannot find a reference to it.

Fix this by explicitly telling LLVM through Clang not to emit bcmp
references. This flag does not need to be behind 'cc-option' because all
working versions of Clang support this flag.

[1]: https://github.com/llvm/llvm-project/commit/8e16d73346f8091461319a7dfc4ddd18eedcff13
[2]: https://travis-ci.com/ClangBuiltLinux/continuous-integration/builds/104027249

Link: https://github.com/ClangBuiltLinux/linux/issues/416
Link: https://bugs.llvm.org/show_bug.cgi?id=41035
Cc: stable@vger.kernel.org
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Change-Id: Ide8b905b0406ef698916f07d728c7d5fd72978e2