[Darwin, PPC] Do not use longcall for 64b code.

[Darwin, PPC] Do not use longcall for 64b code.

The linker [ld64] that supports 64Bit does not need the JBSR longcall
optimisation, and will not work with the most generic case (where the
symbol is undefined external, but there is no symbl stub).  So switch
the longcall option off.  ld64 will generate branch islands as needed.

2019-06-27  Iain Sandoe  <iain@sandoe.co.uk>

	* config/rs6000/rs6000.c (darwin_rs6000_override_options): Do not
	use longcall for 64b code.

From-SVN: r272758