Suppress warnings in certain configuration and fix x86_64 build

See merge request !12

[PowerPC64] Modify register defines in gcm-hash.asm

See merge request !10

* ecc-secp384r1.c (ecc_secp384r1_inv): New function, modular
inverse using powering.
(_nettle_secp_384r1): Analogous updates. Increases signing
performance roughly 15% on x86_64.

* ecc-secp224r1.c (ecc_secp224r1_inv): New function, modular
inverse using powering.
(_nettle_secp_224r1): Analogous updates. Increases signing
performance roughly 17% on x86_64.

* ecc-secp256r1.c (ecc_secp256r1_inv): New function, modular
inverse using powering.
(_nettle_secp_256r1): Analogous updates. Increases signing
performance roughly 6% on x86_64.

* ecc-secp521r1.c (ecc_secp521r1_inv): New function, modular
inverse using powering.
(_nettle_secp_521r1): Analogous updates. Increases signing
performance roughly 15% on x86_64.

* ecc-secp192r1.c (ecc_secp192r1_inv): New function, modular
inverse using powering.
(_nettle_secp_192r1): Use it for p.invert, and also update
h_to_a_itch. Increases signing performance roughly 25% on x86_64.

After these changes, both curve25519 and curve448 need 4*size for
invert and 6*size for sqrt.