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>

include/sysemu/arch_init.h

@@ -24,6 +24,7 @@ enum {
     QEMU_ARCH_TRICORE = (1 << 16),
     QEMU_ARCH_NIOS2 = (1 << 17),
     QEMU_ARCH_HPPA = (1 << 18),
+    QEMU_ARCH_RISCV = (1 << 19),
 };
 
 extern const uint32_t arch_type;