How to build shared library?

i compiled shardc from source in NDK r22b，and followed official guide below:
https://developer.android.com/ndk/guides/graphics/shader-compilers

CLI:
step1: `cd ${NDK_ROOT}/sources/third_party/shaderc/ `
step2: `ndk-build NDK_PROJECT_PATH=. APP_BUILD_SCRIPT=Android.mk APP_STL:=c++_shared APP_ABI=all libshaderc_combined`

after a long time building ,i just got arm64\arm32 & x86\x64 static lib: libsharedc.a

Dir tree of shaderc/libs is:

What's wrong with my CLI param ? or dynamic lib is unsupported? i tried to read Android.mk but it's a little bit complex to me.