Using -DCMAKE_SYSROOT to pass wasi-sysroot properly under msys2 bash

With the following building commands:
```
call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" amd64
set MSYS2_PATH_TYPE=inherit
set MSYSTEM=MINGW32
set CC=cl
set CXX=cl
set "HOME=%CD%"
bash.exe --login -c "make package"
bash.exe --login -c "make check"
cmd /k
```

Fixes following error:
c:\wasi-sdk\bin\clang++.exe --target=wasm32-wasi -DNDEBUG -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -Iinclude/c++build -fdebug-prefix-map=C:\work\wasi-sdk=wasisdk;\v13.5g73d30af917c4 --sysroot=c;C:\CI-Tools\msys64\wasi-sdk\share\wasi-sysroot -fPIC -DLIBCXX_BUILDING_LIBCXXABI -IC:/work/wasi-sdk/build/libcxx/include/c++/v1 -std=c++20 -MD -MT src/CMakeFiles/cxx_static.dir/regex.cpp.obj -MF src\CMakeFiles\cxx_static.dir\regex.cpp.obj.d -o src/CMakeFiles/cxx_static.dir/regex.cpp.obj -c C:/work/wasi-sdk/src/llvm-project/libcxx/src/regex.cpp
In file included from C:/work/wasi-sdk/src/llvm-project/libcxx/src/regex.cpp:9:
In file included from C:/work/wasi-sdk/build/libcxx/include/c++/v1\regex:766:
In file included from C:/work/wasi-sdk/build/libcxx/include/c++/v1\__debug:14:
In file included from C:/work/wasi-sdk/build/libcxx/include/c++/v1\iosfwd:98:
C:/work/wasi-sdk/build/libcxx/include/c++/v1\wchar.h:119:15: fatal error: 'wchar.h' file not found
#include_next <wchar.h>
              ^~~~~~~~~

Makefile

@@ -99,6 +99,7 @@ build/compiler-rt.BUILT: build/llvm.BUILT build/wasi-libc.BUILT
 	# Do the build, and install it.
 	mkdir -p build/compiler-rt
 	cd build/compiler-rt && cmake -G Ninja \
+		-DCMAKE_SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot \
 		-DCMAKE_C_COMPILER_WORKS=ON \
 		-DCMAKE_CXX_COMPILER_WORKS=ON \
 		-DCMAKE_AR=$(BUILD_PREFIX)/bin/ar \
@@ -112,7 +113,7 @@ build/compiler-rt.BUILT: build/llvm.BUILT build/wasi-libc.BUILT
 		-DCOMPILER_RT_ENABLE_IOS=OFF \
 		-DCOMPILER_RT_DEFAULT_TARGET_ONLY=On \
 		-DWASI_SDK_PREFIX=$(BUILD_PREFIX) \
-		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
+		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP)" \
 		-DLLVM_CONFIG_PATH=$(ROOT_DIR)/build/llvm/bin/llvm-config \
 		-DCOMPILER_RT_OS_DIR=wasi \
 		-DCMAKE_INSTALL_PREFIX=$(PREFIX)/lib/clang/$(CLANG_VERSION)/ \
@@ -154,8 +155,9 @@ build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUI
 	# Do the build.
 	mkdir -p build/libcxx
 	cd build/libcxx && cmake -G Ninja $(LIBCXX_CMAKE_FLAGS) \
-	    -DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
-	    -DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
+		-DCMAKE_SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot \
+		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) \
+		-DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) \
 		-DLIBCXX_LIBDIR_SUFFIX=$(ESCAPE_SLASH)/wasm32-wasi \
 		$(LLVM_PROJ_DIR)/libcxx
 	ninja $(NINJA_FLAGS) -v -C build/libcxx
@@ -195,8 +197,9 @@ build/libcxxabi.BUILT: build/libcxx.BUILT build/llvm.BUILT
 	# Do the build.
 	mkdir -p build/libcxxabi
 	cd build/libcxxabi && cmake -G Ninja $(LIBCXXABI_CMAKE_FLAGS) \
-	    -DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
-	    -DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) --sysroot=$(BUILD_PREFIX)/share/wasi-sysroot" \
+		-DCMAKE_SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot \
+		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) \
+		-DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) \
 		-DLIBCXXABI_LIBDIR_SUFFIX=$(ESCAPE_SLASH)/wasm32-wasi \
 		$(LLVM_PROJ_DIR)/libcxxabi
 	ninja $(NINJA_FLAGS) -v -C build/libcxxabi