kbuild: clang: remove crufty HOSTCFLAGS

BugLink: https://bugs.launchpad.net/bugs/1810958

commit df16aaac upstream.

When compiling with `make CC=clang HOSTCC=clang`, I was seeing warnings
that clang did not recognize -fno-delete-null-pointer-checks for HOSTCC
targets.  These were added in commit 61163efa ("kbuild: LLVMLinux:
Add Kbuild support for building kernel with Clang").

Clang does not support -fno-delete-null-pointer-checks, so adding it to
HOSTCFLAGS if HOSTCC is clang does not make sense.

It's not clear why the other warnings were disabled, and just for
HOSTCFLAGS, but I can remove them, add -Werror to HOSTCFLAGS and compile
with clang just fine.
Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
[nc: Adjust context]
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

Makefile

@@ -306,11 +306,6 @@ HOSTCXX      = g++
 HOSTCFLAGS   = -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer -std=gnu89
 HOSTCXXFLAGS = -O2
 
-ifeq ($(shell $(HOSTCC) -v 2>&1 | grep -c "clang version"), 1)
-HOSTCFLAGS  += -Wno-unused-value -Wno-unused-parameter \
-		-Wno-missing-field-initializers -fno-delete-null-pointer-checks
-endif
-
 # Decide whether to build built-in, modular, or both.
 # Normally, just do built-in.