[dev.link] cmd/internal/obj: trim trailing zeros for content hashing
The symbol's data in the object file (sym.P) may already not
contain trailing zeros (e,g, for [10]int{1}), but sometimes it
does (e.g. for [10]int{1,0}). The linker can already handle this
case. We just always trim the trailing zeros for content hashing,
so it can deduplicate [10]int{1} and [10]int{1,0}.
Note: in theory we could just trim the zeros in the symbol data
as well. But currently the linker depends on reading symbol data
for certain symbols (e.g. type symbol decoding), and trimming
will complicates things in the linker.