selftests: vDSO: don't hard-code location of vDSO sources

Architectures use different location for vDSO sources:

	arch/mips/vdso
	arch/sparc/vdso
	arch/arm64/kernel/vdso
	arch/riscv/kernel/vdso
	arch/csky/kernel/vdso
	arch/x86/um/vdso
	arch/x86/entry/vdso
	arch/powerpc/kernel/vdso
	arch/arm/vdso
	arch/loongarch/vdso

Don't hard-code vdso sources location in selftest Makefile. Instead
create a vdso/ symbolic link in tools/arch/$arch/ and update Makefile
accordingly.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tools/arch/x86/vdso

+../../../arch/x86/entry/vdso/
\ No newline at end of file

tools/testing/selftests/vDSO/Makefile

@@ -40,7 +40,7 @@ $(OUTPUT)/vdso_test_getrandom: parse_vdso.c
 $(OUTPUT)/vdso_test_getrandom: CFLAGS += -isystem $(top_srcdir)/tools/include \
                                          -isystem $(top_srcdir)/include/uapi
 
-$(OUTPUT)/vdso_test_chacha: $(top_srcdir)/arch/$(ARCH)/entry/vdso/vgetrandom-chacha.S
+$(OUTPUT)/vdso_test_chacha: $(top_srcdir)/tools/arch/$(ARCH)/vdso/vgetrandom-chacha.S
 $(OUTPUT)/vdso_test_chacha: CFLAGS += -idirafter $(top_srcdir)/tools/include \
                                       -isystem $(top_srcdir)/arch/$(ARCH)/include \
                                       -isystem $(top_srcdir)/include \