CMake: clean up WASI SDK sysroot paths (#39275)

Paths to WASI SDK sysroot should not assume `/share/wasi-sysroot` directory hierarchy. It's much more flexible to have this part hidden under the more general `SWIFT_WASI_SYSROOT_PATH` variable.

Author: MaxDesiatov

cmake/modules/SwiftConfigureSDK.cmake

@@ -378,10 +378,10 @@ macro(configure_sdk_unix name architectures)
         if(NOT arch STREQUAL wasm32)
           message(FATAL_ERROR "unsupported arch for WebAssembly: ${arch}")
         endif()
-        set(SWIFT_SDK_WASI_ARCH_wasm32_PATH "${SWIFT_WASI_SDK_PATH}/share/wasi-sysroot")
+        set(SWIFT_SDK_WASI_ARCH_wasm32_PATH "${SWIFT_WASI_SYSROOT_PATH}")
         set(SWIFT_SDK_WASI_ARCH_wasm32_TRIPLE "wasm32-unknown-wasi")
-        set(SWIFT_SDK_WASI_ARCH_wasm32_LIBC_INCLUDE_DIRECTORY "${SWIFT_WASI_SDK_PATH}/share/wasi-sysroot/include")
-        set(SWIFT_SDK_WASI_ARCH_wasm32_LIBC_ARCHITECTURE_INCLUDE_DIRECTORY "${SWIFT_WASI_SDK_PATH}/share/wasi-sysroot/include")
+        set(SWIFT_SDK_WASI_ARCH_wasm32_LIBC_INCLUDE_DIRECTORY "${SWIFT_WASI_SYSROOT_PATH}/include")
+        set(SWIFT_SDK_WASI_ARCH_wasm32_LIBC_ARCHITECTURE_INCLUDE_DIRECTORY "${SWIFT_WASI_SYSROOT_PATH}/include")
       else()
         message(FATAL_ERROR "unknown Unix OS: ${prefix}")
       endif()