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>

configure

@@ -6797,6 +6797,16 @@ case "$target_name" in
     echo "TARGET_ABI32=y" >> $config_target_mak
     gdb_xml_files="power64-core.xml power-fpu.xml power-altivec.xml power-spe.xml power-vsx.xml"
   ;;
+  riscv32)
+    TARGET_BASE_ARCH=riscv
+    TARGET_ABI_DIR=riscv
+    mttcg=yes
+  ;;
+  riscv64)
+    TARGET_BASE_ARCH=riscv
+    TARGET_ABI_DIR=riscv
+    mttcg=yes
+  ;;
   sh4|sh4eb)
     TARGET_ARCH=sh4
     bflt="yes"
@@ -6966,6 +6976,9 @@ for i in $ARCH $TARGET_BASE_ARCH ; do
   ppc*)
     disas_config "PPC"
   ;;
+  riscv)
+    disas_config "RISCV"
+  ;;
   s390*)
     disas_config "S390"
   ;;