gdb: Fix installation of gcore.1 on some platforms

gdb: Fix installation of gcore.1 on some platforms

gcore is installed on NetBSD, FreeBSD, Solaris (HAVE_NATIVE_GCORE_HOST=1)
and Linux (HAVE_NATIVE_GCORE_TARGET=1). However, even though gcore.1
is installed conditional on those variables, HAVE_NATIVE_GCORE_HOST
was missing from gdb/doc/Makefile.in, so manual installation was
skipped on NetBSD, FreeBSD, and Solaris.

gdb/doc/ChangeLog:
2020-10-06  Michael Forney  <mforney@mforney.org>

        * Makefile.in (HAVE_NATIVE_GCORE_HOST): Add for gcore.1
        install condition.

Change-Id: I17c3ce2ecdfb806ece17f05ba78356b25ffa865e