cmd/link: split off 'Dynimp' string fields to reduce sym.Symbol size
The linker's sym.Symbol struct contains two string fields, "Dynimplib"
and "Dynimpvers" that are used only in very specific circumstances
(for many symbols, such as DWARF syms, they are wasted space). Split
these two off into a separate struct, then point to an instance of
that struct when needed. This reduces the size of sym.Symbol so as to
save space in the common case.
Updates #26186
Change-Id: Id9c74824e78423a215c8cbc105b72665525a1eff
Reviewed-on: https://go-review.googlesource.com/121916 Reviewed-by: Ian Lance Taylor <iant@golang.org>