Only use enable-ec_nistp_64_gcc_128 on amd64 since it requires a
64-bit little-endian architecture and fix the build on !amd64

ecp_nistp224.c:43:9: error: unknown type name 'uint128_t'
typedef uint128_t uint128_t; /* nonstandard; implemented by gcc on 64-bit
^

PR: 220403
Reported by: dewayne@heuristicsystems.com.au
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D11436
MFH: 2017Q3