cmd/7g: fix linux/arm64 build

REGRT1 and REGRT2 are also reserved on arm64 for runtime (duffzero
and duffcopy).

Change-Id: If098527a7f29d16f94bdcec05fd55950b9076e35
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7977Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/7g/gsubr.go

@@ -45,6 +45,8 @@ var unmappedzero int64 = 4096
 var resvd = []int{
 	arm64.REGTMP,
 	arm64.REGG,
+	arm64.REGRT1,
+	arm64.REGRT2,
 	arm64.REG_R31, // REGZERO and REGSP
 	arm64.FREGZERO,
 	arm64.FREGHALF,