[dev.link] cmd/link: clear lib.Textp2 after use

[dev.link] cmd/link: clear lib.Textp2 after use

lib.Textp2 is used to assemble the global Textp2. It is not used
after that point. Free some memory.

Slightly reduces allocation: for linking cmd/compile,
Linksetup_GC     1.10MB ± 0%    0.84MB ± 0%  -23.43%  (p=0.008 n=5+5)

Change-Id: Iec4572e282655306d5ff3e490f8855d479e45acf
Reviewed-on: https://go-review.googlesource.com/c/go/+/228481
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>