RISC-V Build Infrastructure

This adds RISC-V into the build system enabling the following targets:

- riscv32-softmmu
- riscv64-softmmu
- riscv32-linux-user
- riscv64-linux-user

This adds defaults configs for RISC-V, enables the build for the RISC-V
CPU core, hardware, and Linux User Emulation. The 'qemu-binfmt-conf.sh'
script is updated to add the RISC-V ELF magic.

Expected checkpatch errors for consistency reasons:

ERROR: line over 90 characters
FILE: scripts/qemu-binfmt-conf.sh

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>

arch_init.c

@@ -71,6 +71,8 @@ int graphic_depth = 32;
 #define QEMU_ARCH QEMU_ARCH_OPENRISC
 #elif defined(TARGET_PPC)
 #define QEMU_ARCH QEMU_ARCH_PPC
+#elif defined(TARGET_RISCV)
+#define QEMU_ARCH QEMU_ARCH_RISCV
 #elif defined(TARGET_S390X)
 #define QEMU_ARCH QEMU_ARCH_S390X
 #elif defined(TARGET_SH4)