Fix for mi-reverse.exp

Fix for mi-reverse.exp

This test fails on PPC64 because PPC64 prints the value of 3.5 with
more significant digits than on Intel. The patch updates the regular
expression to allow for more significant digits on the constant.

gdb/testsuite/ChangeLog

	* gdb.mi/mi-reverse.exp: mi_execute_to exec-step reverse add check
	for additional digits.