Use "MS" for .debug_str

Use "MS" for .debug_str

I changed my system linker to 'mold', but then I saw some gdb test
failures.  This patch fixes a subset of the failures.

dw2-strp.exp was failing, and investigating showed that there were two
.debug_str sections.  I tracked this down to the .S file not using the
correct section flags.

This patch fixes this problem, plus the other instances I could find.
(Strangely, these did not all cause problems, however.)  I also
changed the DWARF assembler to always use these flags for .debug_str.