Shim exports are called from GuestToHostThunk which dictates the calling
convention. The default system calling convention is different depending
on OS (Windows vs. everything else) and architecture. PR #1339 addresses
this for x64 Linux.
There is no reason for explicit `__cdecl`. Also, it is not available in
GCC. We could use `__attribute__((ms_abi))` or
`__attribute__((sysv_abi))` but that just adds complexity.