libzstd: Explicitly define ZSTD_DISABLE_ASM

On FreeBSD, ZSTD_ASM_SUPPORTED is defined as 0, but on macOS and Linux
it is defined as 1, yet we don't build any of the assembly sources.
Rather than add them just for bootstrapping on non-FreeBSD, explicitly
define ZSTD_DISABLE_ASM so they're not needed and everything is
consistent.

This fixes building a bootstrap LLVM toolchain on non-FreeBSD amd64 (the
only architecture with assembly available).

Reviewed by: emaste, imp
Differential Revision: https://reviews.freebsd.org/D41543