rs6000: Enable more sibcalls when TOC is not preserved

rs6000: Enable more sibcalls when TOC is not preserved

A function compiled with the PC-relative addressing model does not
require r2 to contain a TOC pointer, and does not guarantee that r2
will be preserved for its caller.  Such a function can make sibcalls
without restriction based on TOC preservation rules.  However, a
caller that does preserve r2 cannot make a sibcall to a callee that
does not.

2020-08-19  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-logue.c (rs6000_decl_ok_for_sibcall):
	Sibcalls are always legal when the caller doesn't preserve r2.

gcc/testsuite/
	* gcc.target/powerpc/pcrel-sibcall-1.c: Adjust.