add rv64 backend

It is mostly complete, but still has a few ABI bugs when passing
floats in structs, or when structs are passed partly in register,
and partly on stack.

rv64/all.h

@@ -0,0 +1,49 @@
+#include "../all.h"
+
+typedef struct Rv64Op Rv64Op;
+
+enum Rv64Reg {
+	/* caller-save */
+	T0 = RXX + 1, T1, T2, T3, T4, T5,
+	A0, A1, A2, A3, A4, A5, A6, A7,
+
+	/* callee-save */
+	S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11,
+
+	/* globally live */
+	FP, SP, GP, TP, RA, T6,
+
+	/* FP caller-save */
+	FT0, FT1, FT2, FT3, FT4, FT5, FT6, FT7, FT8, FT9, FT10, FT11,
+	FA0, FA1, FA2, FA3, FA4, FA5, FA6, FA7,
+
+	/* FP callee-save */
+	FS0, FS1, FS2, FS3, FS4, FS5, FS6, FS7, FS8, FS9, FS10, FS11,
+
+	NFPR = FS11 - FT0 + 1,
+	NGPR = T6 - T0 + 1,
+	NGPS = A7 - T0 + 1,
+	NFPS = FA7 - FT0 + 1,
+	NCLR = (S11 - S1 + 1) + (FS11 - FS0 + 1),
+};
+MAKESURE(reg_not_tmp, FS11 < (int)Tmp0);
+
+struct Rv64Op {
+	char imm;
+};
+
+/* targ.c */
+extern int rv64_rsave[];
+extern int rv64_rclob[];
+extern Rv64Op rv64_op[];
+
+/* abi.c */
+bits rv64_retregs(Ref, int[2]);
+bits rv64_argregs(Ref, int[2]);
+void rv64_abi(Fn *);
+
+/* isel.c */
+void rv64_isel(Fn *);
+
+/* emit.c */
+void rv64_emitfn(Fn *, FILE *);