selftests: vDSO: look for arch-specific function name in getrandom test

Don't hard-code x86 specific names. Rather, use vdso_config definitions
to find the correct function matching the architecture.

Add random VDSO function names in names[][]. Remove the #ifdef
CONFIG_VDSO32, as having the name there all the time is harmless and
guaranties a steady index for following strings.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[Jason: add [6] to variable declaration rather than each usage site.]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>