Fixes PR41828. Before this, LLD always emitted SafeSEH chunks and
defined __safe_se_handler_table & size. Now, /safeseh:no leaves those
undefined.
Additionally, we were checking for the safeseh @feat.00 flag in two
places: once to emit errors, and once during safeseh table construction.
The error was set up to be off by default, but safeseh is supposed to be
on by default. I combined the two checks, so now LLD emits an error if
an input object lacks @feat.00 and safeseh is enabled. This caused the
majority of 32-bit LLD tests to fail, since many test input object files
lack @feat.00 symbols. I explicitly added -safeseh:no to those tests to
preserve behavior.
Finally, LLD no longer sets IMAGE_DLL_CHARACTERISTICS_NO_SEH if any
input file wasn't compiled for safeseh.
Should the default be off in the mingw case, if linking objects from gcc?