[POWER10] __morestack calls from pcrel code

Compiling gcc/testsuite/gcc.dg/split-*.c and others with -mcpu=power10
and linking with a non-pcrel libgcc results in crashes due to the
power10 pcrel code not having r2 set for the generic-morestack.c
functions called from __morestack.  There is also a problem when
non-pcrel code calls a pcrel libgcc.  See the patch comments.

A similar situation theoretically occurs with ELFv1 multi-toc
executables, when __morestack might be located in a different toc
group to its caller.  This patch makes no attempt to fix that, since
the gold linker does not support multi-toc (gold is needed for proper
support of -fsplit-stack code) nor does gcc emit __morestack calls
that support multi-toc.

	* config/rs6000/morestack.S (R2_SAVE): Define.
	(__morestack): Save and restore r2.  Set up r2 for called
	functions.

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