Previously, both crypto/dh and crypto/ec defined |TOBN| macros that did
the same thing, but which took their arguments in the opposite order.
This change makes the code consistently use the same macro. It also
makes |STATIC_BIGNUM| available for internal use outside of crypto/bn.
Change-Id: Ide57f6a5b74ea95b3585724c7e1a630c82a864d9
Reviewed-on: https://boringssl-review.googlesource.com/6528
Reviewed-by: Adam Langley <agl@google.com>
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