gdb/testsuite: fix testing gdb.base/skip-inline.exp with clang

gdb/testsuite: fix testing gdb.base/skip-inline.exp with clang

When testing gdb.base/skip-inline.exp using clang, we get failures
when trying to step out of functions, since clang requires one fewer
step when compared to gcc.  The inferior gets increasingly out of sync
as the test continues because of this difference, which generates those
failures.

This commit fixes this by switching those hardcoded steps to
gdb_step_until, to guarantee that the inferior is always synced to what
the test expects.  This approach does not work for the parts that use
step 2 or step 3, so when we identify that clang is being used, those
tests are skipped.