ld: Add -no-pie

gold has had this option for many years. Not having this option caused
some confusion to users.  The help message clarifies the default state.

ld/
    * ldlex.h (enum option_values): Add OPTION_NO_PIE.
    * lexsup.c (struct ld_options): Add -no-pie.
    (parse_args): Handle -no-pie.
    * ld.texi: Document -no-pie.