RISC-V Linux User Emulation

Implementation of linux user emulation for RISC-V.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Sagar Karandikar <sagark@eecs.berkeley.edu>
Signed-off-by: Michael Clark <mjc@sifive.com>

target/riscv/cpu_user.h

@@ -0,0 +1,13 @@
+#define xRA 1   /* return address (aka link register) */
+#define xSP 2   /* stack pointer */
+#define xGP 3   /* global pointer */
+#define xTP 4   /* thread pointer */
+
+#define xA0 10  /* gpr[10-17] are syscall arguments */
+#define xA1 11
+#define xA2 12
+#define xA3 13
+#define xA4 14
+#define xA5 15
+#define xA6 16
+#define xA7 17  /* syscall number goes here */