misc/cgo/testshared: increase size limit in size check

Recently in Fedora we switched binutils ld's separate-code on. This
led to increased size of binaries, especially on 64k aligned arches.
For example trivial test binary size grew from 80k to 211k on ppc64le
tripping the size check(RHBZ#2030308). Therefore adjusting the size limit.

Change-Id: Ic722d90c338739c0b285f40b12ba4d675e9626a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/371634


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>