selftests: default to host arch for LLVM builds

Align the behavior for gcc and clang builds by interpreting unset
`ARCH` and `CROSS_COMPILE` variables in `LLVM` builds as a sign that the
user wants to build for the host architecture.

This patch preserves the properties that setting the `ARCH` variable to an
unknown value will trigger an error that complains about insufficient
information, and that a set `CROSS_COMPILE` variable will override the
target triple that is determined based on presence/absence of `ARCH`.

When compiling with clang, i.e., `LLVM` is set, an unset `ARCH` variable in
combination with an unset `CROSS_COMPILE` variable, i.e., compiling for
the host architecture, leads to compilation failures since `lib.mk` can
not determine the clang target triple. In this case, the following error
message is displayed for each subsystem that does not set `ARCH` in its
own Makefile before including `lib.mk` (lines wrapped at 75 chrs):

  make[1]: Entering directory '/mnt/build/linux/tools/testing/selftests/
   sysctl'
  ../lib.mk:33: *** Specify CROSS_COMPILE or add '--target=' option to
   lib.mk.  Stop.
  make[1]: Leaving directory '/mnt/build/linux/tools/testing/selftests/
   sysctl'

In the same scenario a gcc build would default to the host architecture,
i.e., it would use plain `gcc`.

Fixes: 795285e ("selftests: Fix clang cross compilation")
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Valentin Obst <kernel@valentinobst.de>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/lib.mk

@@ -7,6 +7,8 @@ else ifneq ($(filter -%,$(LLVM)),)
 LLVM_SUFFIX := $(LLVM)
 endif
 
+CLANG := $(LLVM_PREFIX)clang$(LLVM_SUFFIX)
+
 CLANG_TARGET_FLAGS_arm          := arm-linux-gnueabi
 CLANG_TARGET_FLAGS_arm64        := aarch64-linux-gnu
 CLANG_TARGET_FLAGS_hexagon      := hexagon-linux-musl
@@ -18,7 +20,13 @@ CLANG_TARGET_FLAGS_riscv        := riscv64-linux-gnu
 CLANG_TARGET_FLAGS_s390         := s390x-linux-gnu
 CLANG_TARGET_FLAGS_x86          := x86_64-linux-gnu
 CLANG_TARGET_FLAGS_x86_64       := x86_64-linux-gnu
-CLANG_TARGET_FLAGS              := $(CLANG_TARGET_FLAGS_$(ARCH))
+
+# Default to host architecture if ARCH is not explicitly given.
+ifeq ($(ARCH),)
+CLANG_TARGET_FLAGS := $(shell $(CLANG) -print-target-triple)
+else
+CLANG_TARGET_FLAGS := $(CLANG_TARGET_FLAGS_$(ARCH))
+endif
 
 ifeq ($(CROSS_COMPILE),)
 ifeq ($(CLANG_TARGET_FLAGS),)
@@ -30,7 +38,7 @@ else
 CLANG_FLAGS     += --target=$(notdir $(CROSS_COMPILE:%-=%))
 endif # CROSS_COMPILE
 
-CC := $(LLVM_PREFIX)clang$(LLVM_SUFFIX) $(CLANG_FLAGS) -fintegrated-as
+CC := $(CLANG) $(CLANG_FLAGS) -fintegrated-as
 else
 CC := $(CROSS_COMPILE)gcc
 endif # LLVM