breakpoint-in-ro-region.exp: Support targets stopping in mid-line after "si"

breakpoint-in-ro-region.exp: Support targets stopping in mid-line after "si"

On some targets each of the assignments "i = 0" in the C source for
"breakpoint-in-ro-region.exp" are compiled to a single instruction.
Then each "si" stops at the beginning of the next source line.  But on
some other targets (like s390) such an assignment compiles to multiple
instructions.  Then "si" may stop in mid-line, and GDB displays the PC
address in addition to the source line number.  This was not considered
by the regexp for this case.

gdb/testsuite/ChangeLog:

	* gdb.base/breakpoint-in-ro-region.exp (test_single_step): In the
	regexps for GDB's current line display, accept a hex address
	preceding the line number.