invoke.texi: Add documentation for -muninit-const-in-rodata.

invoke.texi: Add documentation for -muninit-const-in-rodata.

	* invoke.texi: Add documentation for -muninit-const-in-rodata.
	* config/mips/mips.h (MASK_UNINIT_CONST_IN_RODATA): Define.
	(TARGET_UNINIT_CONST_IN_RODATA): Define.
	(text_section): Add switches -munint-const-in-rodata and
	-mno-uninit-const-in-rodata.
	(ASM_OUTPUT_COMMON): Remove.
	(ASM_OUTPUT_ALIGNED_DECL_COMMON): Define. Check if uninitialized
	const objects should be placed in read-only data. Otherwise declare
	them in common.

From-SVN: r30562