ld: handle Plan 9 ar format

ld: handle Plan 9 ar format

The Go version has 64 character long section names; originally,
in Plan 9, the limit was 16.  To provide compatibility, this
change allows the input length to be either the target length
or the earlier option.	The section name is extended with spaces
where required.

This has been tested to work without regressions in the
Go environment, testing the older alternative has not been
possible yet.

R=rsc
CC=golang-dev
https://golang.org/cl/4650071