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 <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>

tools/testing/selftests/vDSO/Makefile

···
 TEST_GEN_PROGS += vdso_test_chacha
 endif
 
-CFLAGS := -std=gnu99
+CFLAGS := -std=gnu99 -O2
 
 ifeq ($(CONFIG_X86_32),y)
 LDLIBS += -lgcc_s