[RS6000] PR97107, libgo fails to build for power10

[RS6000] PR97107, libgo fails to build for power10

Calls from split-stack code to non-split-stack code need to expand
mapped stack memory via __morestack.  Even tail calls.

__morestack is quite a surprising function on powerpc in that it calls
back to its caller, and a tail call will continue running in the
context of extra mapped stack.

	PR target/97107
	* config/rs6000/rs6000-internal.h (struct rs6000_stack): Improve
	calls_p comment.
	* config/rs6000/rs6000-logue.c (rs6000_stack_info): Likewise.
	(rs6000_expand_split_stack_prologue): Emit the prologue for
	functions that make a sibling call.