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>

hw/riscv/Makefile.objs

@@ -0,0 +1,11 @@
+obj-y += riscv_htif.o
+obj-y += riscv_hart.o
+obj-y += sifive_e.o
+obj-y += sifive_clint.o
+obj-y += sifive_prci.o
+obj-y += sifive_plic.o
+obj-y += sifive_test.o
+obj-y += sifive_u.o
+obj-y += sifive_uart.o
+obj-y += spike.o
+obj-y += virt.o