[RFA] Use mkinstalldirs not mkdir in mklibgcc

[DJ Delorie]

> Hmmm, not a good idea:
> 
> for d in libgcc; do \
>   if [ -d $d ]; then true; else "/bin/sh ../../gcc/gcc/mkinstalldirs"
> $d; fi \
> done
> /bin/sh: /bin/sh ../../gcc/gcc/mkinstalldirs: No such file or directory

No, not ..."'$mk'"..., I meant ...'"$mk"'...

I don't want the double quotes to appear in the output file, I just
want the echo that *creates* the output file to be protected.