Corrects lib lookup in case-sensitive OSes

Downcase name of Windows import libraries to avoid duplicates references
(Eg. kernel32 and Kernel32) but to correctly find the libraries on
case-sensitive filesystems, like when cross-compiling from Linux.

While on Windows `Kernel32` and `kernel32` result in the correct library
found for linking, it does not work the same way when cross-compiling.

This also applies to Winsock2 (ws2_32) usage.

The change normalizes on the usage of downcased names for these
libraries (which will have no impact when compiling natively on
Windows).

src/lib_c/x86_64-windows-msvc/c/libloaderapi.cr

@@ -1,6 +1,6 @@
 require "c/winnt"
 
-@[Link("Kernel32")]
+@[Link("kernel32")]
 lib LibC
   alias FARPROC = Void*
 

src/lib_c/x86_64-windows-msvc/c/winsock2.cr

@@ -3,7 +3,7 @@ require "./basetsd"
 require "./guiddef"
 require "./winbase"
 
-@[Link("WS2_32")]
+@[Link("ws2_32")]
 lib LibC
   alias SOCKET = UINT_PTR