[LLD][COFF] Gate second-dot section-name stripping on MinGW (#199625)
The comment in getOutputSectionName has always called the second-dot
stripping "for MinGW" (e.g. .ctors.NNNN), but the code applied it on
every target. This hiddes a split-dwarf bug #199616.
Take an isMinGW gate and skip the stripping when it is false.