Fix linking with lld by marking OPENSSL_armcap_P as hidden.

Linking with lld fails as it contains a relative address, however the data
this address is for may be relocated from the shared object to the main
executable.

Fix this by adding the hidden attribute. This stops moving this value to
the main executable. It seems this is implicit upstream as it uses a
version script.

Approved by: jkim
Sponsored by: DARPA, AFRL