gdb: Let gdbserver be empty for riscv64

gdbserver does not yet build for riscv64, therefore
let the package be empty so we can build core-image-sato-sdk-ptest

(From OE-Core rev: 111f65528e3c0b147c1713cfc684958f94c41cd6)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/gdb/gdb.inc

@@ -8,3 +8,6 @@ inherit gettext
 # cross-canadian must not see this
 PACKAGES =+ "gdbserver"
 FILES_gdbserver = "${bindir}/gdbserver"
+
+ALLOW_EMPTY_gdbserver_riscv64 = "1"
+