cmd/ld, cmd/6l, cmd/8l, cmd/5l: fix hidden/local symbol import for ELF systems

cmd/ld, cmd/6l, cmd/8l, cmd/5l: fix hidden/local symbol import for ELF systems
   Introduce a newsym() to cmd/lib.c to add a symbol but don't add
them to hash table.
   Introduce a new bit flag SHIDDEN and bit mask SMASK to handle hidden
and/or local symbols in ELF symbol tables. Though we still need to order
the symbol table entries correctly.
   Fix for issue 3261 comment #9.
   For CL 5822049.

R=iant, rsc
CC=golang-dev
https://golang.org/cl/5823055