Building GHC with debug info results in broken executables on HEAD+Windows.

GHC itself builds fine it seems. But when we build timeout.exe for the testsuite timeout.exe fails with lots of errors of this form:

*** unexpected failure for T16282(normal)
Compile failed (exit code 1) errors were:
timeout.exe: allocaBytesAligned: invalid argument (alignment (=104) must be a power of two!)

This might have been broken for a while. But allocaBytesAligned only started checking if the alignment argument is a power of two very recently (yesterday). It does seem to work on linux.