Replace pc component with unknown in stdlib/CMakeLists.txt

Author: MaxDesiatov

stdlib/public/CMakeLists.txt

@@ -194,8 +194,10 @@ if(SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB)
     list(APPEND EMBEDDED_STDLIB_TARGET_TRIPLES
       "i686    i686-unknown-none-elf       i686-unknown-none-elf"
       "x86_64  x86_64-unknown-none-elf     x86_64-unknown-none-elf"
-      "i686    i686-pc-windows-coff       i686-pc-windows-coff"
-      "x86_64  x86_64-pc-windows-coff     x86_64-pc-windows-coff"
+      # Without specifying the `windows` component LLVM refuses to
+      # generate valid COFF object files.
+      "i686    i686-unknown-windows-coff   i686-unknown-windows-coff"
+      "x86_64  x86_64-unknown-windows-coff x86_64-unknown-windows-coff"
     )
   endif()