gdb/testsuite: fix testing gdb.reverse/step-reverse.exp with clang

gdb/testsuite: fix testing gdb.reverse/step-reverse.exp with clang

When testing using reverse-stepi to fully step through a function, the
code checks for an infinite loop by seeing if we land on the line that
contains the return statement multiple times. This assumption only works
if there is only one instruction associated with that line, which is how
GCC handles line information, but other compilers may handle it differently.
Clang-15, for instance, associates 6. Because of this, the inferior used
to get seriously out of sync with the test expectations, and result in 13
spurious failures. The same issue occurs with gdb.reverse/step-precsave.exp.

This commit changes the test so that we check for PC instead of line
number. The test still only happens when the same line is detected, to
simplify the resulting log. With this change, no new failures are
emitted when using clang.

Approved-By: Tom Tromey <tom@tromey.com>