Cygwin: Fix warnings about narrowing conversions of NTSTATUS constants

Cygwin: Fix warnings about narrowing conversions of NTSTATUS constants

Fix warnings with gcc 12 about narrowing conversions of NTSTATUS
constants when used as case labels, e.g:

> ../../../../src/winsup/cygwin/exceptions.cc: In static member function ‘static int exception::handle(EXCEPTION_RECORD*, void*, CONTEXT*, PDISPATCHER_CONTEXT)’:
> ../../../../src/winsup/cygwin/exceptions.cc:670:10: error: narrowing conversion of ‘-1073741682’ from ‘NTSTATUS’ {aka ‘int’} to ‘unsigned int’ [-Wnarrowing]

See also: c5bdf60ac46401a51a7e974333d9622966e22d67

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>