selftests: vDSO: build tests with O2 optimization

Without -O2, the generated code for testing chacha function is awful.
GCC even implements rol32() as a function of 20 instructions instead of
just using the rotlwi instruction.

	~# time ./vdso_test_chacha
	TAP version 13
	1..1
	ok 1 chacha: PASS
	real    0m 37.16s
	user    0m 36.89s
	sys     0m 0.26s

Several other selftests directory add -O2, and the kernel is also
always built with optimisation active. Do the same for vDSO selftests.

With this patch the time is reduced by approximately 15%.

	~# time ./vdso_test_chacha
	TAP version 13
	1..1
	ok 1 chacha: PASS
	real    0m 32.09s
	user    0m 31.86s
	sys     0m 0.22s

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