[dev.link] cmd/link: fix accidental escape in extreloc

[dev.link] cmd/link: fix accidental escape in extreloc

We accidentally passed the address of a local to a function
pointer, where we should pass the address of a global.

Linking cmd/compile with external linking:

Asmb2_GC       32.5ms ± 5%    21.6ms ± 3%  -33.57%  (p=0.016 n=5+4)
Asmb2_GC       29.2MB ± 0%     6.4MB ± 0%  -78.20%  (p=0.008 n=5+5)
Asmb2_GC        1.43M ± 0%     0.00M ± 4%  -99.98%  (p=0.008 n=5+5)

Change-Id: I4754189bcc20f824627d95858ba35285d53c614d
Reviewed-on: https://go-review.googlesource.com/c/go/+/245337
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>