bsp/tms570: Add -mbe32 to LINKFLAGS

There is not just big-endian on ARM.  We have two variants BE32
(obsolete) and BE8.  The Cortex-R5F processor supports only BE8,
however, some TMS570 variants are BE32 internally.  In GCC 8 and later,
the --be8 option is passed to the linker based on the selected
architecture or CPU.  Use BE32 by default for the TMS570 BSP.

In GCC, see:

commit 63d03dcecdafe34715282a5155cfc2162375feca
Author: Richard Earnshaw <rearnsha@arm.com>
Date:   Mon Jul 3 13:22:05 2017 +0000

     [arm] Clean up generation of BE8 format images.

spec/build/bsps/arm/tms570/grp.yml

@@ -38,6 +38,8 @@ links:
   uid: optscibaud
 - role: build-dependency
   uid: opttms570ls3137
+- role: build-dependency
+  uid: optlinkflags
 - role: build-dependency
   uid: ../../linkcmds
 - role: build-dependency

spec/build/bsps/arm/tms570/optlinkflags.yml

@@ -0,0 +1,17 @@
+SPDX-License-Identifier: CC-BY-SA-4.0 OR BSD-2-Clause
+actions:
+- get-string: null
+- split: null
+- env-append: LINKFLAGS
+build-type: option
+copyrights:
+- Copyright (C) 2022 embedded brains GmbH (http://www.embedded-brains.de)
+default:
+- -mbe32
+default-by-variant: []
+description: |
+  TMS570-specific flags passed to the linker.
+enabled-by: true
+links: []
+name: TMS570_LINKFLAGS
+type: build