selftests: vDSO: separate LDLIBS from CFLAGS for libsodium

On systems that set -Wl,--as-needed, putting the -lsodium in the wrong
place on the command line means we get a linker error:

      CC       vdso_test_chacha
    /usr/bin/ld: /tmp/ccKpjnSM.o: in function `main':
    vdso_test_chacha.c:(.text+0x276): undefined reference to `crypto_stream_chacha20'
    collect2: error: ld returned 1 exit status

Fix this by passing pkg-config's --libs output to the LDFLAGS field
instead of the CFLAGS field, as is customary.
Reported-by: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tools/testing/selftests/vDSO/Makefile

 # SPDX-License-Identifier: GPL-2.0
 uname_M := $(shell uname -m 2>/dev/null || echo not)
 ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/x86/ -e s/x86_64/x86/)
-SODIUM := $(shell pkg-config --libs --cflags libsodium 2>/dev/null)
+SODIUM_LIBS := $(shell pkg-config --libs libsodium 2>/dev/null)
+SODIUM_CFLAGS := $(shell pkg-config --cflags libsodium 2>/dev/null)
 
 TEST_GEN_PROGS := vdso_test_gettimeofday
 TEST_GEN_PROGS += vdso_test_getcpu
@@ -13,7 +14,7 @@ endif
 TEST_GEN_PROGS += vdso_test_correctness
 ifeq ($(uname_M),x86_64)
 TEST_GEN_PROGS += vdso_test_getrandom
-ifneq ($(SODIUM),)
+ifneq ($(SODIUM_LIBS),)
 TEST_GEN_PROGS += vdso_test_chacha
 endif
 endif
@@ -41,8 +42,9 @@ $(OUTPUT)/vdso_test_getrandom: CFLAGS += -isystem $(top_srcdir)/tools/include \
                                          -isystem $(top_srcdir)/include/uapi
 
 $(OUTPUT)/vdso_test_chacha: $(top_srcdir)/tools/arch/$(ARCH)/vdso/vgetrandom-chacha.S
+$(OUTPUT)/vdso_test_chacha: LDLIBS += $(SODIUM_LIBS)
 $(OUTPUT)/vdso_test_chacha: CFLAGS += -idirafter $(top_srcdir)/tools/include \
                                       -idirafter $(top_srcdir)/arch/$(ARCH)/include \
                                       -idirafter $(top_srcdir)/include \
                                       -D__ASSEMBLY__ -DBULID_VDSO -DCONFIG_FUNCTION_ALIGNMENT=0 \
-                                      -Wa,--noexecstack $(SODIUM)
+                                      -Wa,--noexecstack $(SODIUM_CFLAGS)