gdb/testsuite: Add a proc to test where compiler links the epilogue

gdb/testsuite: Add a proc to test where compiler links the epilogue

Different compilers link the epilogue of functions to different lines.
As an example, gcc links it to the closing brace of the function,
whereas clang links it to the last statement of the function.  This
difference is important for the testsuite, since the where GDB will land
after a step can be wildly different.  Where possible, this dependency
should be side-stepped in the testsuite, but it isn't always possible,
so this commit adds a gdb_caching_proc that is able to detect where the
epilogue is linked, so tests can react accordingly.