Remove the arg0 field from struct amd64_frame. Its existence was a bug,

since on amd64 the first argument to a function is generally not on the
stack.

Revert an old DTrace bug fix to some code that assumed that
sizeof(struct amd64_frame) == 16.

Reviewed by:	jhb, kib
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D3255

sys/cddl/dev/dtrace/amd64/dtrace_isa.c

@@ -440,7 +440,7 @@ dtrace_getarg(int arg, int aframes)
 	}
 
 	arg -= (inreg + 1);
-	stack = (uintptr_t *)fp + 2;
+	stack = (uintptr_t *)&fp[1];
 
 load:
 	DTRACE_CPUFLAG_SET(CPU_DTRACE_NOFAULT);

sys/x86/include/stack.h

@@ -45,7 +45,6 @@ struct i386_frame {
 struct amd64_frame {
 	struct amd64_frame	*f_frame;
 	u_long			f_retaddr;
-	u_long			f_arg0;
 };
 
 struct i386_frame {