Added a simpler startup/EpicsHostArch for backwards-compatibility

Many downstream modules seem to be using the EpicsHostArch script in
their CI build scripts and would break if we remove that. I created
a new version that finds and execs the Perl script directly.

Also reworded and expanded the Release Notes about the EpicsHostArch
scripts.

startup/EpicsHostArch

@@ -0,0 +1,21 @@
+#!/bin/sh
+# Script to find and run the Perl EpicsHostArch.pl script.
+
+# This script is provided for backwards-compatibility only and may be
+# dropped from future releases of Base. Please adjust callers to run
+# the Perl version directly as this startup directory isn't copied to
+# INSTALL_LOCATION by the EPICS build system.
+
+EHA=EpicsHostArch.pl
+
+cd "$(dirname "$0")/.."
+
+# Perl script will be installed into lib/perl
+[[ -f lib/perl/$EHA ]] && exec perl lib/perl/$EHA $*
+
+# If Base hasn't been built yet, use the source Luke
+[[ -f src/tools/$EHA ]] && exec perl src/tools/$EHA $*
+
+# Die with an error message
+echo "$0: Can't find $EHA" >&2
+exit 1