Compilation Failure on `GOOS=aix`

[mohammed90]

We have received a report of compilation failure with GOOS is aix (see caddyserver/caddy#5970). The build failure reports the following:

# github.com/google/certificate-transparency-go/x509
../../../../go/pkg/mod/github.com/google/certificate-transparency-go@v1.1.6/x509/cert_pool.go:65:9: undefined: loadSystemRoots
../../../../go/pkg/mod/github.com/google/certificate-transparency-go@v1.1.6/x509/root.go:21:32: undefined: loadSystemRoots
../../../../go/pkg/mod/github.com/google/certificate-transparency-go@v1.1.6/x509/verify.go:759:12: c.systemVerify undefined (type *Certificate has no field or method systemVerify)

The 3 errors listed in the error message report functions/methods that are defined in root_unix.go. The build tags exclude AIX. I believe the extensive list can be replaced with the new unix build tag (implemented here golang/go@c3fcd01) that will include any Unix-like system by default. This avoids such hassles in the future. FWIW, AIX supports SSL_CERT_DIR and SSL_CERT_FILE if you prefer to be explicit about the supported OS.

[roger2hk]

The caddyserver/caddy has a indirect dependency on this repo. Note that the x509 folder in this repo is a weaken forked version of the go/crypto/x509. According to the dependency tree, I will create an issue or a pull request in google/go-tspi to see if there is a strong reason to rely on this repo's x509 version.

[roger2hk]

The issue (golang/go#30416) to support for parsing certificates with RSAES-OAEP public keys in crypto/x509 is still open. 😕

https://github.com/google/go-tspi/blob/68e5e7a4206b89249f71ec492f15c19edd3a5061/verification/verification.go#L140-L145