Use volatile pointers when attempting to trigger SIGSEGVs

Use volatile pointers when attempting to trigger SIGSEGVs

Clang fails to compile a number of files with the following warning:
indirection of non-volatile null pointer will be deleted, not trap
[-Wnull-dereference].  This commit qualifies the relevant pointers
with 'volatile'.

gdb/testsuite/ChangeLog:

	* gdb.base/bigcore.c (main): Use a volatile pointer when
	attempting to trigger a SIGSEGV.
	* gdb.base/gcore-relro-pie.c (break_here): Likewise.
	* gdb.base/gcore-tls-pie.c (break_here): Likewise.
	* gdb.base/savedregs.c (thrower): Likewise.
	* gdb.mi/mi-syn-frame.c (bar): Likewise.