add arm build support (#3)

.gitignore

@@ -1 +1,2 @@
 build
+build-*/

build-arm-linux-gnueabihf.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+set -x
+
+mkdir -p build-arm-linux-gnueabihf
+cd build-arm-linux-gnueabihf
+cmake -DCMAKE_TOOLCHAIN_FILE=../toolchains/arm-linux-gnueabihf.toolchain.cmake ..
+make VERBOSE=1 -j4
+make install

toolchains/README.md

@@ -0,0 +1,11 @@
+# gcc-arm-8.3-2019.03-x86_64-arm-linux-gnueabihf.tar.xz
+
+Go to <https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-a/downloads/8-3-2019-03> to download the toolchain.
+
+```bash
+mkdir /ceph-fj/fangjun/software
+cd /ceph-fj/fangjun/software
+tar xvf /path/to/gcc-arm-8.3-2019.03-x86_64-arm-linux-gnueabihf.tar.xz
+
+export PATH=/ceph-fj/fangjun/software/gcc-arm-8.3-2019.03-x86_64-arm-linux-gnueabihf/bin:$PATH
+```

toolchains/arm-linux-gnueabihf.toolchain.cmake

@@ -0,0 +1,17 @@
+# Copied from https://github.com/Tencent/ncnn/blob/master/toolchains/arm-linux-gnueabihf.toolchain.cmake
+set(CMAKE_SYSTEM_NAME Linux)
+set(CMAKE_SYSTEM_PROCESSOR arm)
+
+set(CMAKE_C_COMPILER "arm-linux-gnueabihf-gcc")
+set(CMAKE_CXX_COMPILER "arm-linux-gnueabihf-g++")
+
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+
+set(CMAKE_C_FLAGS "-march=armv7-a -mfloat-abi=hard -mfpu=neon")
+set(CMAKE_CXX_FLAGS "-march=armv7-a -mfloat-abi=hard -mfpu=neon")
+
+# cache flags
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}" CACHE STRING "c flags")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}" CACHE STRING "c++ flags")