rs6000: mark clobber for registers changed by untpyed_call

rs6000: mark clobber for registers changed by untpyed_call

As PR93047 said, __builtin_apply/__builtin_return does not work well with
-frename-registers.  This is caused by return register(e.g. r3) is used to
rename another register, before return register is stored to stack.
This patch fix this issue by emitting clobber for those egisters which
maybe changed by untyped call.

gcc/
2020-02-26  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR target/93047
	* config/rs6000/rs6000.md (untyped_call): Add emit_clobber.

gcc/testsuite
2020-02-26  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR target/93047
	* gcc.dg/torture/stackalign/builtin-return-2.c: New test case.