arm.md (sibcall, [...]): Add RETURN as part of the pattern, remove clobber of LR.

* arm.md (sibcall, sibcall_value): Add RETURN as part of the pattern,
remove clobber of LR.
(sibcall_insn, sibcall_value_insn): Update accordingly.
(sibcall_epilogue): Remove debugging comment from assembler stream.

From-SVN: r55836