debug: fix reading -gdwarf generated debug sections in COFF files

I had accidentally regressed support for -gdwarf in 461fb499f3cff9038a427eae120fb34defc9ab38 when I changed the logic to
use the already-mapped exe/dll image instead of loading it from disk. The string table is mapped as all zeroes by the loader,
so if a section header's name is longer than 8 bytes (like the ones generated by -gdwarf), then the name can't be read.

Now, if any section headers require the string table, the file is mapped from disk.

windows: Add NtCreateSection/NtMapViewOfSection/NtUnmapViewOfSection