rs6000: -mreadonly-in-sdata (PR82411)

rs6000: -mreadonly-in-sdata (PR82411)

This adds a new option -mreadonly-in-sdata (on by default) that
controls whether readonly data can be put in sdata.  (For EABI this
does nothing, readonly data is put in sdata2 as usual).

	PR target/82411
	* config/rs6000/rs6000.c (rs6000_elf_in_small_data_p): Don't put
	readonly data in sdata, if that is disabled.
	* config/rs6000/sysv4.opt (mreadonly-in-sdata): New option.
	* doc/invoke.texi (RS/6000 and PowerPC Options): Document
	-mreadonly-in-sdata option.

gcc/testsuite/
	PR target/82411
	* gcc.target/powerpc/ppc-sdata-2.c: Skip if -mno-readonly-in-sdata.

From-SVN: r258340