Allow truss to operate on ELFv2 processes.

Summary:
Currently, truss doesn't work on ELFv2 processes due to not recognizing the ABI.

Since there's no special handling needed here, just adding a PROCABI struct for
it is sufficient to get it working.

Submitted by:	git_bdragon.rtk0.net
Differential Revision: https://reviews.freebsd.org/D18352

usr.bin/truss/powerpc64-freebsd.c

@@ -117,3 +117,14 @@ static struct procabi powerpc64_freebsd = {
 };
 
 PROCABI(powerpc64_freebsd);
+
+static struct procabi powerpc64_freebsd_elfv2 = {
+	"FreeBSD ELF64 V2",
+	SYSDECODE_ABI_FREEBSD,
+	powerpc64_fetch_args,
+	powerpc64_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(powerpc64_freebsd_elfv2.extra_syscalls),
+	{ NULL }
+};
+
+PROCABI(powerpc64_freebsd_elfv2);