cmd/internal/obj: convert fields of LSym from uint8 to bool

cmd/internal/obj: convert fields of LSym from uint8 to bool

No performance regression measurable:

name      old time/op    new time/op    delta
Template     432ms ± 3%     422ms ± 2%  -2.34%   (p=0.010 n=10+9)
GoTypes      1.46s ± 1%     1.46s ± 1%    ~     (p=0.796 n=10+10)
Compiler     7.15s ± 1%     7.14s ± 1%    ~      (p=0.447 n=10+9)

Change-Id: I21b93cb989017b6fec2215de2423d87f25cf538c
Reviewed-on: https://go-review.googlesource.com/21220
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>