makefiles/tools/gdb.inc.mk: prefer $(target)-gdb over gdb-multiarch

In an ideal world everyone would just install `gdb-multiarch` and be
happy. However, some MCUs need magic GDB versions sprinkled with
unicorn-stardust-Espressif-patches...

Since there is little reason to have `$(target)-gdb` installed in
addition to `gdb-multiarch` if `gdb-multiarch` would work fine, let's
assume the user wants to use `$(target)-gdb` when present over
`gdb-multiarch`.

Co-authored-by: Gunar Schorcht <gunar@schorcht.net>
(cherry picked from commit b9b63da)

makefiles/tools/gdb.inc.mk

@@ -1,7 +1,11 @@
-# new versions of gdb will support all architectures in one binary
-ifeq ($(shell gdb-multiarch -v > /dev/null 2>&1; echo $$?),0)
-export GDB        ?= gdb-multiarch
+export GDBPREFIX ?= $(PREFIX)
+
+# If the user installed a magic single target GDB rather than just using
+# gdb-multiarch, there typically is a reason for it - e.g. missing support for
+# that particular target in gdb-multiarch.
+ifeq ($(shell $(GDBPREFIX)gdb -v > /dev/null 2>&1; echo $$?),0)
+  export GDB ?= $(GDBPREFIX)gdb
 else
-export GDBPREFIX  ?= $(PREFIX)
-export GDB        ?= $(GDBPREFIX)gdb
+  # gdb-multiarch is normally
+  export GDB ?= gdb-multiarch
 endif