amd64: avoid reading past end of passed struct

If the size of the struct is not a multiple of 8, the actual struct
size may be different from the size reserved on the stack.

This fixes the case where the struct is passed in memory, but we
still may over-read a struct passed in registers. A TODO is added
for now.

arm64/abi.c

@@ -144,6 +144,7 @@ sttmps(Ref tmp[], int cls[], uint nreg, Ref mem, Fn *fn)
 	}
 }
 
+/* todo, may read out of bounds */
 static void
 ldregs(int reg[], int cls[], int n, Ref mem, Fn *fn)
 {