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.