selftests: vDSO: unconditionally build chacha test

Rather than using symlinks to find the vgetrandom-chacha.S file for each
arch, store this in a file that uses the compiler to determine
architecture, and then make use of weak symbols to skip the test on
architectures that don't provide the code.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>