Fix gdb.cp/step-and-next-inline.exp with Clang

Clang fails to compile gdb.cp/step-and-next-inline.cc, with the
following error:

  clang-12: error: unknown argument: '-gstatement-frontiers'
  compiler exited with status 1

This commit fixes the testcase by only passing -gstatement-frontiers
when building with GCC.  This commit also alters two checks marked as
known failures, to mark them as known failures only when built using
GCC.

gdb/testsuite/ChangeLog:

	* gdb.cp/step-and-next-inline.exp: Only require
	-gstatement-frontiers when building with GCC.
	Only setup KFAIL's for GCC issues when using
	a GCC-built executable.