Add RISC-V support.

This supports both 0.11 and 0.13 versions of the debug spec.

Support for `-rtos riscv` will come in a separate commit since it was
easy to separate out, and is likely to be more controversial.

Flash support for the SiFive boards will also come in a later commit.

Change-Id: I1d38fe669c2041b4e21a5c54a091594aac3e2190
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4578
Tested-by: jenkins
Reviewed-by: Liviu Ionescu <ilg@livius.net>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

src/target/Makefile.am

@@ -4,7 +4,9 @@ else
 OOCD_TRACE_FILES =
 endif
 
-%C%_libtarget_la_LIBADD = %D%/openrisc/libopenrisc.la
+%C%_libtarget_la_LIBADD = %D%/openrisc/libopenrisc.la \
+	%D%/riscv/libriscv.la
+
 
 STARTUP_TCL_SRCS += %D%/startup.tcl
 
@@ -218,3 +220,4 @@ INTEL_IA32_SRC = \
 	%D%/arm_cti.h
 
 include %D%/openrisc/Makefile.am
+include %D%/riscv/Makefile.am