add new target-specific abi0 pass

The general idea is to give abis a
chance to talk before we've done all
the optimizations. Currently, all
targets eliminate {par,arg,ret}{sb,ub,...}
during this pass. The forthcoming
arm64_apple will, however, insert
proper extensions during abi0.

Moving forward abis can, for example,
lower small-aggregates passing there
so that memory optimizations can
interact better with function calls.

rv64/targ.c

@@ -44,7 +44,8 @@ Target T_rv64 = {
 	.retregs = rv64_retregs,
 	.argregs = rv64_argregs,
 	.memargs = rv64_memargs,
-	.abi = rv64_abi,
+	.abi0 = elimsb,
+	.abi1 = rv64_abi,
 	.isel = rv64_isel,
 	.emitfn = rv64_emitfn,
 	.emitfin = elf_emitfin,