Adjust wasip3 defaults

* Don't build the target by default yet
* Build a separate compiler-rt for updated ABI eventually

CMakeLists.txt

@@ -11,7 +11,7 @@ list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 project(wasi-sdk)
 include(ExternalProject)
 
-set(WASI_SDK_TARGETS "wasm32-wasi;wasm32-wasip1;wasm32-wasip2;wasm32-wasip3;wasm32-wasip1-threads;wasm32-wasi-threads"
+set(WASI_SDK_TARGETS "wasm32-wasi;wasm32-wasip1;wasm32-wasip2;wasm32-wasip1-threads;wasm32-wasi-threads"
   CACHE STRING "List of WASI targets to build")
 option(WASI_SDK_BUILD_TOOLCHAIN "Build a toolchain instead of the sysroot" OFF)
 

cmake/wasi-sdk-sysroot.cmake

@@ -101,9 +101,20 @@ function(define_compiler_rt target)
   add_dependencies(compiler-rt-build compiler-rt-build-${target})
 endfunction()
 
+# The `compiler-rt` for `wasm32-wasip1` will be reused for `wasm32-wasip2` and
+# `wasm32-wasi`. The version for `wasm32-wasip1-threads` will be reused for
+# `wasm32-wasi-threads`. Different builds are needed for different codegen flags
+# and such across the threaded/not target.
 define_compiler_rt(wasm32-wasip1)
 define_compiler_rt(wasm32-wasip1-threads)
 
+# If a p3 target is requested, also build compiler-rt for that target. WASIp3
+# will eventually have a different ABI than wasm32-wasip2, so this separate
+# build is needed.
+if(WASI_SDK_TARGETS MATCHES p3)
+  define_compiler_rt(wasm32-wasip3)
+endif()
+
 # In addition to the default installation of `compiler-rt` itself also copy
 # around some headers and make copies of the `wasi` directory as `wasip1` and
 # `wasip2` and `wasip3`
@@ -134,7 +145,6 @@ add_dependencies(compiler-rt-post-build compiler-rt-build)
 
 add_custom_target(compiler-rt DEPENDS compiler-rt-build compiler-rt-post-build)
 
-
 # =============================================================================
 # wasi-libc build logic
 # =============================================================================