rs6000: Small varargs for BE SVR4 (PR61729, PR77850)

rs6000: Small varargs for BE SVR4 (PR61729, PR77850)

The varargs code for SVR4 puts all (integer) arguments in 4-byte slots.
When it then reads an item from there as something not a multiple of 4
bytes, it needs to adjust the address if big endian.  We didn't yet do
that.

This fixes the g++.dg/abi/scoped1.C, gcc.dg/compat/scalar-by-value-4,
and gcc.dg/compat/scalar-return-4 testcases.

	PR target/61729
	PR target/77850
	* config/rs6000/rs6000.c (rs6000_gimplify_va_arg): Adjust address to
	read from, for big endian.

From-SVN: r244740