build-compiler-rt: Make the CMAKE_C_COMPILER_TARGET vendor field consistent

Currently, there's no practical difference if the spelling of this
triple is inconsistent, but if we'd enable e.g. the per-target
runtime directories, the exact spelling does make a difference.

For consistency, pass CMAKE_C_COMPILER_TARGET with the exact right
spelling everywhere.

build-compiler-rt.sh

@@ -112,7 +112,7 @@ for arch in $ARCHS; do
         -DCMAKE_RANLIB="$PREFIX/bin/llvm-ranlib" \
         -DCMAKE_C_COMPILER_WORKS=1 \
         -DCMAKE_CXX_COMPILER_WORKS=1 \
-        -DCMAKE_C_COMPILER_TARGET=$arch-windows-gnu \
+        -DCMAKE_C_COMPILER_TARGET=$arch-w64-windows-gnu \
         -DCOMPILER_RT_DEFAULT_TARGET_ONLY=TRUE \
         -DCOMPILER_RT_USE_BUILTINS_LIBRARY=TRUE \
         -DCOMPILER_RT_BUILD_BUILTINS=$BUILD_BUILTINS \