Make `GFp_armcap_P`/`ring_core_*_OPENSSL_armcap_P` a hidden symbol

[briansmith]

When one tries to create a --crate-type=staticlib or similar, GFp_armcap_P will be exported from the staticlib. If that static library is then linked into Rust program that depends on ring, the user will get "duplicate symbol" errors for this symbol. The hypothesis is that this is due to the symbol not having visibility=hidden.

This symbol is special because it is prefixed_export! which means it gets a #[export_name = "..."] annotation. Other symbols aren't so affected on ARM because they are defined in C and are given visibility=hidden.

On non-ARM/Aarch64/x86/x86-64 targets, bn_mul_mont is affected in the same way for the same reason. That should be rectified as well. Basically we need to remove prefixed_export! and all its uses.

[briansmith]

Related Rust issues/enhancement requests:

• Ability from the top-level of the compilation not to mark #[no_mangle] items exported from shared library rust-lang/rust#73958
• Don't leak non-exported symbols from staticlibs rust-lang/rust#104707
• Linking error on Rust 1.70 aarch64-unknown-linux-musl toolchain rust-lang/rust#113104

[briansmith]

PR #1823 attempts to address this.

[briansmith]

Fixed (I speculate) in 0.17.6 by PR #1823. It would be great to get confirmation that this is indeed fixed.