selftests/bpf: Pass target triple to get_sys_includes macro

When cross-compiling [1], the get_sys_includes make macro should use
the target system include path, and not the build hosts system include
path.

Make clang honor the CROSS_COMPILE triple.

[1] e.g. "ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- make"
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Link: https://lore.kernel.org/bpf/20221115182051.582962-2-bjorn@kernel.org

tools/testing/selftests/bpf/Makefile

@@ -310,9 +310,9 @@ $(RESOLVE_BTFIDS): $(HOST_BPFOBJ) | $(HOST_BUILD_DIR)/resolve_btfids	\
 # Use '-idirafter': Don't interfere with include mechanics except where the
 # build would have failed anyways.
 define get_sys_includes
-$(shell $(1) -v -E - </dev/null 2>&1 \
+$(shell $(1) $(2) -v -E - </dev/null 2>&1 \
 	| sed -n '/<...> search starts here:/,/End of search list./{ s| \(/.*\)|-idirafter \1|p }') \
-$(shell $(1) -dM -E - </dev/null | grep '__riscv_xlen ' | awk '{printf("-D__riscv_xlen=%d -D__BITS_PER_LONG=%d", $$3, $$3)}')
+$(shell $(1) $(2) -dM -E - </dev/null | grep '__riscv_xlen ' | awk '{printf("-D__riscv_xlen=%d -D__BITS_PER_LONG=%d", $$3, $$3)}')
 endef
 
 # Determine target endianness.
@@ -320,7 +320,11 @@ IS_LITTLE_ENDIAN = $(shell $(CC) -dM -E - </dev/null | \
 			grep 'define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__')
 MENDIAN=$(if $(IS_LITTLE_ENDIAN),-mlittle-endian,-mbig-endian)
 
-CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG))
+ifneq ($(CROSS_COMPILE),)
+CLANG_TARGET_ARCH = --target=$(notdir $(CROSS_COMPILE:%-=%))
+endif
+
+CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG),$(CLANG_TARGET_ARCH))
 BPF_CFLAGS = -g -Werror -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN) 		\
 	     -I$(INCLUDE_DIR) -I$(CURDIR) -I$(APIDIR)			\
 	     -I$(abspath $(OUTPUT)/../usr/include)
@@ -542,7 +546,7 @@ $(eval $(call DEFINE_TEST_RUNNER,test_progs,no_alu32))
 # Define test_progs BPF-GCC-flavored test runner.
 ifneq ($(BPF_GCC),)
 TRUNNER_BPF_BUILD_RULE := GCC_BPF_BUILD_RULE
-TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(call get_sys_includes,gcc)
+TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(call get_sys_includes,gcc,)
 $(eval $(call DEFINE_TEST_RUNNER,test_progs,bpf_gcc))
 endif