Pass -Wno-deprecated-register for gdb.cp that use "register"

Clang fails to compile three testcases with the following error:
  warning: 'register' storage class specifier is deprecated and
    incompatible with C++17 [-Wdeprecated-register]

This prevents the following testcases from executing:
  gdb.cp/classes.exp
  gdb.cp/inherit.exp
  gdb.cp/misc.exp

This commit builds those testcases with -Wno-deprecated-register, to
avoid the failure.  Note that this commit reveals five "wrong access
specifier for typedef" failures in gdb.cp/classes.exp when compiling
the testsuite with clang.

gdb/testsuite/ChangeLog:

	* gdb.cp/classes.exp (prepare_for_testing): Add
	additional_flags=-Wno-deprecated-register.
	* gdb.cp/inherit.exp (prepare_for_testing): Likewise.
	* gdb.cp/misc.exp: Likewise.