gdb.trace: Fix unavailable.exp if last register happens to be PC.

gdb.trace: Fix unavailable.exp if last register happens to be PC.

unavailable.exp executes "info registers", expecting to find at least
two instances of "<unavailable>".  However, it uses
"<unavailable>.*<unavailable>" as the pattern, which doesn't match
when the last register happens to be available (eg. PC).  Change it
to ".*<unavailable>.*<unavailable>.*" instead.

Noticed on s390, no regression on x86_64.

gdb/testsuite/ChangeLog:

	* gdb.trace/unavailable.exp (gdb_unavailable_registers_test_1): Fix
	info registers pattern.