testsuite: adapt the testsuite for stdcall removal

This patch adjusts test cases to handle the stdcall removal:

- Some stdcall usages are replaced with ccall since stdcall doesn't
  make sense anymore.
- We also preserve some stdcall usages, and check in the expected
  warning messages to ensure GHC always warn about stdcall usages
  (-Wunsupported-calling-conventions) as expected.
- Error code testsuite coverage is slightly improved,
  -Wunsupported-calling-conventions is now tested.
- Obsolete code paths related to i386 windows are also removed.