Add riscv cross build

- RISCV architecture support with gcc toolchain
- Fix readme for support architecture list

Change-Id: I5ed06362c435e5b1b472179297b3c8ea5ad56a85
Signed-off-by: Kwanghoon Son <kwangson@yahoo.com>
(cherry picked from commit 48c0a5292b2170c81f4f5e3acedca5151f1696e6)

README.md

diff --git a/README.md b/README.md
index 7af05af..0d51080 100644
--- a/README.md
+++ b/README.md
@@ -165,6 +165,8 @@
  - armv7-linux-gcc.cmake
  - armv7-mingw-gcc.cmake
  - armv7s-ios.cmake
+ - ppc-linux-gcc.cmake
+ - riscv-linux-gcc.cmake
  - x86-ios-simulator.cmake
  - x86-linux.cmake
  - x86-macos.cmake

build/cmake/toolchains/riscv-linux-gcc.cmake

diff --git a/build/cmake/toolchains/riscv-linux-gcc.cmake b/build/cmake/toolchains/riscv-linux-gcc.cmake
new file mode 100644
index 0000000..21e7370
--- /dev/null
+++ b/build/cmake/toolchains/riscv-linux-gcc.cmake
@@ -0,0 +1,35 @@
+#
+# Copyright (c) 2022, Alliance for Open Media. All rights reserved
+#
+# This source code is subject to the terms of the BSD 2 Clause License and the
+# Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License was
+# not distributed with this source code in the LICENSE file, you can obtain it
+# at www.aomedia.org/license/software. If the Alliance for Open Media Patent
+# License 1.0 was not distributed with this source code in the PATENTS file, you
+# can obtain it at www.aomedia.org/license/patent.
+#
+if(AOM_BUILD_CMAKE_TOOLCHAINS_RISCV_LINUX_GCC_CMAKE_)
+  return()
+endif() # AOM_BUILD_CMAKE_TOOLCHAINS_RISCV_LINUX_GCC_CMAKE_
+set(AOM_BUILD_CMAKE_TOOLCHAINS_RISCV_LINUX_GCC_CMAKE_ 1)
+
+set(CMAKE_SYSTEM_NAME "Linux")
+
+if("${CROSS}" STREQUAL "")
+
+  # Default the cross compiler prefix to something known to work.
+  set(CROSS riscv64-unknown-linux-gnu-)
+endif()
+
+if(NOT CMAKE_C_COMPILER)
+  set(CMAKE_C_COMPILER ${CROSS}gcc)
+endif()
+if(NOT CMAKE_CXX_COMPILER)
+  set(CMAKE_CXX_COMPILER ${CROSS}g++)
+endif()
+if(NOT AS_EXECUTABLE)
+  set(AS_EXECUTABLE ${CROSS}as)
+endif()
+set(CMAKE_SYSTEM_PROCESSOR "riscv")
+
+set(CONFIG_RUNTIME_CPU_DETECT 0 CACHE STRING "")