riscv: Add semihosting support for user mode

This could made testing more easier and ARM/AArch64 has supported on
their linux user mode too, so I think it should be reasonable.

Verified GCC testsuite with newlib/semihosting.

Signed-off-by: Kito Cheng <kito.cheng@sifive.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210107170717.2098982-7-keithp@keithp.com>
Message-Id: <20210108224256.2321-18-alex.bennee@linaro.org>

linux-user/riscv/cpu_loop.c

@@ -23,6 +23,7 @@
 #include "qemu.h"
 #include "cpu_loop-common.h"
 #include "elf.h"
+#include "hw/semihosting/common-semi.h"
 
 void cpu_loop(CPURISCVState *env)
 {
@@ -91,6 +92,10 @@ void cpu_loop(CPURISCVState *env)
             sigcode = TARGET_SEGV_MAPERR;
             sigaddr = env->badaddr;
             break;
+        case RISCV_EXCP_SEMIHOST:
+            env->gpr[xA0] = do_common_semihosting(cs);
+            env->pc += 4;
+            break;
         case EXCP_DEBUG:
         gdbstep:
             signum = TARGET_SIGTRAP;