Print <unavailable> for unavailable registers in info register output.

Print <unavailable> for unavailable registers in info register output.

  https://sourceware.org/ml/gdb-patches/2013-08/msg00171.html

gdb/ChangeLog

	* infcmd.c (default_print_one_register_info): Use val_print to
	print all values even optimized out or unavailable ones.  Don't
	try to print a raw form of optimized out or unavailable values.

gdb/testsuite/ChangeLog

	* gdb.trace/unavailable.exp (gdb_unavailable_registers_test):
	Expect <unavailable> pattern.