Added the -pic flag to 6c and 6g to avoid assembler instructions that
cannot use RIP-relative adressing. This is needed to support the -shared mode
in 6l.

See also:
https://golang.org/cl/6926049
https://golang.org/cl/6822078

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7064048