Respect aarch64 -march=|-mcpu= flag and check whether -mcpu=native works

Packagers may specify a target system up front, which should be
respected. The CRC extensions are not strictly necessary as there is a
software fallback.

Otherwise, use -mcpu=native if it works (it won't when cross-compiling)
or -mcpu=generic+crc.

Signed-off-by: James Le Cuirot <jlecuirot@microsoft.com>

Author: chewi

src/overlaybd/zfile/CMakeLists.txt

@@ -11,7 +11,14 @@ target_include_directories(crc32_lib PUBLIC
 if (CMAKE_SYSTEM_PROCESSOR STREQUAL x86_64)
     target_compile_options(crc32_lib PUBLIC -msse4.2 -mcrc32)
 else()
-    target_compile_options(crc32_lib PRIVATE -march=native -mcpu=generic+crc)
+    if (NOT CMAKE_CXX_FLAGS MATCHES "-march=|-mcpu=")
+        check_cxx_compiler_flag(-mcpu=native COMPILER_HAS_NATIVE_FLAG)
+        if (COMPILER_HAS_NATIVE_FLAG)
+            target_compile_options(crc32_lib PRIVATE -mcpu=native)
+        else ()
+            target_compile_options(crc32_lib PRIVATE -mcpu=generic+crc)
+        endif ()
+    endif ()
 endif()
 
 if(ENABLE_DSA OR ENABLE_ISAL)