[RISC-V] Support CMake build with custom compiler flags

This CL aims to support custom compiler flags.
Because upstream clang has supported to build for x280 with -mcpu=sifive-x280.

Change-Id: Ic8fbf026fe6805ac5c3422a9ccc3f53293c89570
Signed-off-by: Bruce Lai <bruce.lai@sifive.com>
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/4713191
Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>

docs/getting_started.md

@@ -238,6 +238,18 @@ After running script, clang & qemu are built in `build-toolchain-qemu/riscv-clan
           -DUSE_RVV=ON .
     cmake --build out/Release/
 
+#### Customized Compiler Flags
+
+Customized compiler flags are supported by `-DRISCV_COMPILER_FLAGS="xxx"`.
+If `-DRISCV_COMPILER_FLAGS="xxx"` is manually assigned, other compile flags(e.g disable -march=xxx) will not be appended.
+
+Example:
+
+    cmake -B out/Release/ -DUNIT_TEST=ON \
+          -DCMAKE_BUILD_TYPE=Release \
+          -DCMAKE_TOOLCHAIN_FILE="./riscv_script/riscv-clang.cmake" \
+          -DRISCV_COMPILER_FLAGS="-mcpu=sifive-x280" \
+          .
 
 ### Run on QEMU
 

riscv_script/riscv-clang.cmake

@@ -28,9 +28,11 @@ set(CMAKE_OBJDUMP "${TOOLCHAIN_PATH}/bin/llvm-objdump")
 set(CMAKE_OBJCOPY "${TOOLCHAIN_PATH}/bin/llvm-objcopy")
 
 # compile options
+set(RISCV_COMPILER_FLAGS "" CACHE STRING "Compile flags")
+# if user provides RISCV_COMPILER_FLAGS, appeding compile flags is avoided.
+if(RISCV_COMPILER_FLAGS STREQUAL "")
   message(STATUS "USE_RVV: ${USE_RVV}")
   message(STATUS "USE_AUTO_VECTORIZER: ${USE_AUTO_VECTORIZER}")
-set(RISCV_COMPILER_FLAGS)
   if(USE_RVV)
     list(APPEND RISCV_COMPILER_FLAGS "-march=rv64gcv")
     if(NOT USE_AUTO_VECTORIZER)
@@ -40,6 +42,7 @@ if(USE_RVV)
   else()
     list(APPEND RISCV_COMPILER_FLAGS "-march=rv64gc")
   endif()
+endif()
 message(STATUS "RISCV_COMPILER_FLAGS: ${RISCV_COMPILER_FLAGS}")
 
 set(CMAKE_C_FLAGS             "${RISCV_COMPILER_FLAGS} ${CMAKE_C_FLAGS}")