[RFA] Minor improvement to coremark, avoid unconditional jump to return

[RFA] Minor improvement to coremark, avoid unconditional jump to return

gcc/
	* cfgcleanup.cc (bb_is_just_return): No longer static.
	* cfgcleanup.h (bb_is_just_return): Add prototype.
	* cfgrtl.cc (fixup_reorder_chain): Do not create an
	unconditional jump to a return block.  Conditionally
	remove unreachable blocks.

gcc/testsuite/

	* gcc.target/riscv/ret-1.c: New test.