Makefile: fix clang warnings due to clang support

Building u-boot tools with clang as a host compiler e.g. on
FreeBSD with `gmake HOSTCC=clang CONFIG_USE_PRIVATE_LIBGCC=y tools`
leads to many warnings [1] for every compiler invocation since
commit 598e2d33. Part of mentioned commit imports linux patches:

 - kbuild: LLVMLinux: Adapt warnings for compilation with clang
 - kbuild: LLVMLinux: Add Kbuild support for building kernel with Clang

No version of clang supports the gcc fno-delete-null-pointer-checks
though, but it is only passed to clang. Gcc does not have the clang
specific Qunused-arguments for the target. Furthermore several
warnings are disabled which aren't encountered in u-boot. Since such
a build has worked for quite some time and works after removing these
changes, just remove the clang specific handling to restore normal
building with clang as hostcc.

[1] Actual warnings
-------------------
  GEN     include/autoconf.mk.dep
arm-freebsd-gcc: unrecognized option '-Qunused-arguments'

  HOSTCC  scripts/basic/fixdep
clang: warning: argument unused during compilation:
                '-fno-delete-null-pointer-checks'

cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>