drop -G flag and add target amd64_apple

apple support is more than assembly syntax
in case of arm64 machines, and apple syntax
is currently useless in all cases but amd64;
rather than having a -G option that only
makes sense with amd64, we add a new target
amd64_apple

rv64/targ.c

@@ -47,6 +47,8 @@ Target T_rv64 = {
 	.abi = rv64_abi,
 	.isel = rv64_isel,
 	.emitfn = rv64_emitfn,
+	.emitfin = elf_emitfin,
+	.asloc = ".L",
 };
 
 MAKESURE(rsave_size_ok, sizeof rv64_rsave == (NGPS+NFPS+1) * sizeof(int));