X86: adjust the windows 64 calling convention for Swift

Adjust the Win64 calling convention for Swift to pass self in R13, which
is traditionally a CSR.  This makes the behaviour similar to the SysV CC
for Swift as well.  This should improve the argument passing on Windows,
although it comes at a high cost of ABI incompatibility.  Fortunately in
this case, there is no guarantee of ABI stability, and so we can make
this incompatible change.