[ELF] Teach GNU Driver about --stats.

This is mainly for back-compatibility with GNU ld.
Ideally --stats should be a general option in LinkingContext, providing
individual stats for every pass in the linking process.
In the GNU driver, a better wording could be used, but there's no need
to change it for now.

Differential Revision:	D7657
Reviewed by:	ruiu

llvm-svn: 230157