This will allow a static linker (with -ffunction-sections since things aren't
split into files) to drop unused parts of DH and DSA. Notably, the parameter
generation bits pull in primality-checking code.

Change-Id: I25087e4cb91bc9d0ab43bcb267c2e2c164e56b59
Reviewed-on: https://boringssl-review.googlesource.com/6388
Reviewed-by: Adam Langley <agl@google.com>