cmd/link: use uint32 as symbol index

Currently, a symbol's global index, the Sym type, is defined as an
int, which is 64-bit on 64-bit machines. We're unlikely to have
more than 4 billion symbols in the near future. Even if we will,
we will probably hit some other limit (e.g. section size) before
the symbol number limit. Use a 32-bit type to reduce memory usage.

E,g, linking cmd/compile in external linking mode (on macOS/amd64)

Munmap_GC    43.2M ± 0%     35.5M ± 1%   -17.74%  (p=0.000 n=16+20)

This brings the memory usage back before the previous CL, and even
lower.

Change-Id: Ie185f1586638fe70d8121312bfa9410942d518c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/487416


Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>