gdb/testsuite: Add test to step through function epilogue

The testsuite implicitly tests GDB's ability to step through epilogues
in multiple tests, without doing it explicitly anywhere.  This is
unfortunate, as clang does not emit epilogue information, so using clang
on our testsuite makes many tests fail.  This patch adds a central,
explicit test for walking through the epilogue so we can safely remove
this from other tests and have them working with clang.

The test created attempts to step through a simple epilogue, an
epilogue that ends on another epilogue, and epilogues leading to other
function calls.