lib(private)zstd: Fix riscv build

Link __bswap[ds]i2() intrinsics in to libzstd for riscv, where the C runtime
apparently lacks such intrinsics.

Broken in r330894.

Reported by:	asomers
Sponsored by:	Dell EMC Isilon

lib/libzstd/Makefile

@@ -43,4 +43,11 @@ ZSTDDIR=	${SRCTOP}/sys/contrib/zstd
 CFLAGS.zstd_compress.c+= -O0
 .endif
 
+# Work around lack of __bswap[sd]i2() intrinsics on riscv.  Probably should be
+# added to one of the runtime C libraries instead.
+.if ${MACHINE_CPUARCH} == "riscv"
+.PATH:	${ZSTDDIR}/lib/freebsd
+SRCS+=	zstd_kfreebsd.c
+.endif
+
 .include <bsd.lib.mk>

sys/contrib/zstd/lib/freebsd/zstd_kfreebsd.c

@@ -27,8 +27,13 @@
  */
 
 #include "zstd_kfreebsd.h"
+#include <sys/param.h>
 #include <sys/endian.h>
 
+#ifndef _KERNEL
+#include <strings.h>
+#endif
+
 /*
  * The kernel as a standalone target does not link against libgcc or
  * libcompiler-rt. On platforms (e.g., MIPS and RISCV) that do not have a