Immediate seg-fault on 32-bit windows build

On 32-bit Windows (built with MSYS), GHC crashes almost immediately when used with -prof, -debug, -threaded or any other "way" flag. Ian boiled it down as follows.

It seems that the commits

    0c4a9f38637dfc3bc8fd48e8ba6bf64da51b727b
    ecd967612877e1965ddebefe9b83acd837bb413a

introduced the problem, but I'm not sure whether they are the direct cause or just happened to tickle a pre-existing bug. Attached is a self-contained module that goes wrong:

$ inplace/bin/ghc-stage1 -debug --make test -O -fforce-recomp $ ./test +RTS -DS; echo $?
cap 0: initialised
A
Segmentation fault/access violation in generated code
1

Works with 7.4.1:

$ ghc -debug --make test -O -fforce-recomp $ ./test; echo $?
A
[WayDyn]
B
0

It's quite fragile, e.g. if I remove

    ghcMode               :: (),

from the DynFlags type then it succeeds.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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