selftests/bpf: Add SKIP_LLVM makefile variable

Introduce SKIP_LLVM makefile variable that allows to avoid using llvm
dependencies when building BPF selftests. This is different from
existing feature-llvm, as the latter is a result of automatic detection
and should not be set by user explicitly.
Avoiding llvm dependencies could be useful for environments that do not
have them, given that as of now llvm dependencies are required only by
jit_disasm_helpers.c.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: mykyta5

tools/testing/selftests/bpf/Makefile

@@ -34,6 +34,9 @@ OPT_FLAGS	?= $(if $(RELEASE),-O2,-O0)
 LIBELF_CFLAGS	:= $(shell $(PKG_CONFIG) libelf --cflags 2>/dev/null)
 LIBELF_LIBS	:= $(shell $(PKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
 
+SKIP_DOCS	?=
+SKIP_LLVM	?=
+
 ifeq ($(srctree),)
 srctree := $(patsubst %/,%,$(dir $(CURDIR)))
 srctree := $(patsubst %/,%,$(dir $(srctree)))
@@ -172,6 +175,7 @@ override OUTPUT := $(patsubst %/,%,$(OUTPUT))
 endif
 endif
 
+ifneq ($(SKIP_LLVM),1)
 ifeq ($(feature-llvm),1)
   LLVM_CFLAGS  += -DHAVE_LLVM_SUPPORT
   LLVM_CONFIG_LIB_COMPONENTS := mcdisassembler all-targets
@@ -187,6 +191,7 @@ ifeq ($(feature-llvm),1)
   endif
   LLVM_LDFLAGS += $(shell $(LLVM_CONFIG) --ldflags)
 endif
+endif
 
 SCRATCH_DIR := $(OUTPUT)/tools
 BUILD_DIR := $(SCRATCH_DIR)/build