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>