selftests/bpf: Remove unnecessary link dependencies

Remove llvm dependencies from binaries that do not use llvm libraries.
Filter out libxml2 from llvm dependencies, as it seems that
it is not actually used. This patch reduced link dependencies
for BPF selftests.
The next line was adding llvm dependencies to every target in the
makefile, while the only targets that require those are test
runnners (test_progs, test_progs-no_alu32,...):
```
$(OUTPUT)/$(TRUNNER_BINARY): LDLIBS += $$(LLVM_LDLIBS)
```

Before this change:
ldd linux/tools/testing/selftests/bpf/veristat
linux-vdso.so.1 (0x00007ffd2c3fd000)
libelf.so.1 => /lib64/libelf.so.1 (0x00007fe1dcf89000)
libz.so.1 => /lib64/libz.so.1 (0x00007fe1dcf6f000)
libm.so.6 => /lib64/libm.so.6 (0x00007fe1dce94000)
libzstd.so.1 => /lib64/libzstd.so.1 (0x00007fe1dcddd000)
libxml2.so.2 => /lib64/libxml2.so.2 (0x00007fe1dcc54000)
libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007fe1dca00000)
libc.so.6 => /lib64/libc.so.6 (0x00007fe1dc600000)
/lib64/ld-linux-x86-64.so.2 (0x00007fe1dcfb1000)
liblzma.so.5 => /lib64/liblzma.so.5 (0x00007fe1dc9d4000)
libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fe1dcc38000)

After:
ldd linux/tools/testing/selftests/bpf/veristat
linux-vdso.so.1 (0x00007ffc83370000)
libelf.so.1 => /lib64/libelf.so.1 (0x00007f4b87515000)
libz.so.1 => /lib64/libz.so.1 (0x00007f4b874fb000)
libc.so.6 => /lib64/libc.so.6 (0x00007f4b87200000)
libzstd.so.1 => /lib64/libzstd.so.1 (0x00007f4b87444000)
/lib64/ld-linux-x86-64.so.2 (0x00007f4b8753d000)

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

tools/testing/selftests/bpf/Makefile

···
   # Prefer linking statically if it's available, otherwise fallback to shared
   ifeq ($(shell $(LLVM_CONFIG) --link-static --libs >/dev/null 2>&1 && echo static),static)
     LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --libs $(LLVM_CONFIG_LIB_COMPONENTS))
-    LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --system-libs $(LLVM_CONFIG_LIB_COMPONENTS))
+    LLVM_LDLIBS  += $(filter-out -lxml2,$(shell $(LLVM_CONFIG) --link-static --system-libs $(LLVM_CONFIG_LIB_COMPONENTS)))
     LLVM_LDLIBS  += -lstdc++
   else
     LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-shared --libs $(LLVM_CONFIG_LIB_COMPONENTS))
···
 	$(Q)rsync -aq $$^ $(TRUNNER_OUTPUT)/
 endif
 
-$(OUTPUT)/$(TRUNNER_BINARY): LDLIBS += $$(LLVM_LDLIBS)
-$(OUTPUT)/$(TRUNNER_BINARY): LDFLAGS += $$(LLVM_LDFLAGS)
-
 # some X.test.o files have runtime dependencies on Y.bpf.o files
 $(OUTPUT)/$(TRUNNER_BINARY): | $(TRUNNER_BPF_OBJS)
 
···
 			     $(OUTPUT)/veristat				\
 			     | $(TRUNNER_BINARY)-extras
 	$$(call msg,BINARY,,$$@)
-	$(Q)$$(CC) $$(CFLAGS) $$(filter %.a %.o,$$^) $$(LDLIBS) $$(LDFLAGS) -o $$@
+	$(Q)$$(CC) $$(CFLAGS) $$(filter %.a %.o,$$^) $$(LDLIBS) $$(LLVM_LDLIBS) $$(LDFLAGS) $$(LLVM_LDFLAGS) -o $$@
 	$(Q)$(RESOLVE_BTFIDS) --btf $(TRUNNER_OUTPUT)/btf_data.bpf.o $$@
 	$(Q)ln -sf $(if $2,..,.)/tools/build/bpftool/$(USE_BOOTSTRAP)bpftool \
 		   $(OUTPUT)/$(if $2,$2/)bpftool