tests/tcg/xtensa: allow testing big-endian cores

Don't disable all big-endian tests, instead check whether $(CORE) is
supported by the configured $(QEMU) and enable tests if it is.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index d51ddee..94faa80 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -371,6 +371,7 @@
 F: target/xtensa/
 F: hw/xtensa/
 F: tests/tcg/xtensa/
+F: tests/tcg/xtensaeb/
 F: disas/xtensa.c
 F: include/hw/xtensa/xtensa-isa.h
 F: configs/devices/xtensa*/default.mak