cmd/ld: don't pass -rdynamic to external linker if -static is used
cmd/ld: don't pass -rdynamic to external linker if -static is used

Fixes issue 7800.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/87790051

pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go