random: vDSO: add __arch_get_k_vdso_rng_data() helper for data page access

_vdso_data is specific to x86 and __arch_get_k_vdso_data() is provided
so that all architectures can provide the requested pointer.

Do the same with _vdso_rng_data, provide __arch_get_k_vdso_rng_data()
and don't use x86 _vdso_rng_data directly.

Until now vdso/vsyscall.h was only included by time/vsyscall.c but now
it will also be included in char/random.c, leading to a duplicate
declaration of _vdso_data and _vdso_rng_data.

To fix this issue, move the declaration in a C file. vma.c looks like
the most appropriate candidate. We don't need to replace the definitions
in vsyscall.h by declarations as declarations are already in asm/vvar.h.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>