libcrypto: build nistp* on powerpc64le and riscv64

[emaste]

Summary

libcrypto intends to provide these routines on little-endian 64-bit targets.

Reported by:    ci.freebsd.org
Sponsored by:   The FreeBSD Foundation

Test Plan

make tinderbox in progress

[emaste]

secure/lib/libcrypto/Makefile

233

I'm not sure if ASM_riscv64 gets defined or not

[emaste]

Define ASM_riscv64

[emaste]

secure/lib/libcrypto/Makefile

233

Of course ASM_* isn't really right variable name for something controlling C files, but it's consistent with what's already here, and we may have riscv64 asm files in the future.

[jrtc27]

These should really be gated by ${MACHINE_ABI:Mlittle-endian} && ${MACHINE_ABI:Mlong64}

[emaste]

use @jrtc27's suggestion