filter_args not correctly functioning

[baentsch]

there seems another issue with filter_algs which appears to set everything to OFF except those explicitly provided.
For example, using the STD config I get

#define OQS_ENABLE_SIG_FALCON 1
#define OQS_ENABLE_SIG_falcon_512 1
/* #undef OQS_ENABLE_SIG_falcon_512_avx2 */
/* #undef OQS_ENABLE_SIG_falcon_512_aarch64 */
...

But I'd expect the aarch64-optimized version to be enabled on my laptop, which is the case with the cmake default config.

Originally posted by @bhess in #1702 (comment)

(apologies to @bhess for incorrectly assigning this instead of #1704).