Fix linux.signest test failure on AArch64.

char is unsigned by default on ARM based machines; https://www.arm.linux.org.uk/docs/faqs/signedchar.php. This leads to wrong usage of the signals_pending variable, which is negative when a signal is being handled.

core/globals.h

@@ -912,7 +912,11 @@ struct _dcontext_t {
 
     dr_where_am_i_t whereami; /* where control is at the moment */
 #ifdef UNIX
-    char signals_pending; /* != 0: pending; < 0: currently handling one */
+    /* On ARM based machines, char is unsigned by default.
+     * https://www.arm.linux.org.uk/docs/faqs/signedchar.php
+     * But we need signed char, so we explicitly qualify this declaration.
+     */
+    signed char signals_pending; /* != 0: pending; < 0: currently handling one */
 #endif
 
     /************* end of offset-crucial fields *********************/