Please render package cross-compiler aware

[IfSixWasNine]

Due to using let in hsc2hs files, the hdbc-odbc package cannot be used during cross-compilation. In order to cross-compile it for the aarch64-architecture (MariaDB/ODBC on Debian Buster on Nirogen8M) on an x86_64.architecture (GHC with stack tool on Ubuntu 20.04 LTS multiarch installation), I had to download the package and remove the let uses and with also replacing the "#{CALLCONV}" occurences by "ccall".

The package could simply be rendered cross-compiler aware by using separate function call headers for "stdcall" and "ccall" instead of using let-controlled macros. Please repair the present issue.