libgo: update libtool.m4 to support GNU/Hurd

This patch was incorrectly installed in the GCC repo:
https://gcc.gnu.org/pipermail/gcc-patches/2021-January/562758.html

Change-Id: I12766baf02bfdf2233f1c5bde1a270f06b020aa7
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/321076
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed