cmd/ld: drop gcargs, gclocals symbols from symbol table

cmd/ld: drop gcargs, gclocals symbols from symbol table

Update #6853

Every function now has a gcargs and gclocals symbol
holding associated garbage collection information.
Put them all in the same meta-symbol as the go.func data
and then drop individual entries from symbol table.

Removing gcargs and gclocals reduces the size of a
typical binary by 10%.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/65870044