cmd/link: move ElfType field in sym.Symbol to cold section

The sym.Symbol 'ElfType' field is used only for symbols corresponding
to things in imported shared libraries, hence is not needed in the
common case. Relocate it to sym.AuxSymbol so as to shrink the main
Symbol struct.

Updates #26186

Change-Id: I803efc561c31a0ca1d93eca434fda1c862a7b2c5
Reviewed-on: https://go-review.googlesource.com/125479


Reviewed-by: Ian Lance Taylor <iant@golang.org>