Fix "noinit" attribute being ignored for -O0 and -fdata-sections

Fix "noinit" attribute being ignored for -O0 and -fdata-sections

Variables with the "noinit" attribute are ignored at -O0 because they
are treated like a regular bss variable and placed in the .bss section.

With -fdata-sections they are ignored because they are not handled in
resolve_unique_section.

gcc/ChangeLog:

	* tree.h (DECL_NOINIT_P): Define.
	* varasm.c (DECL_NOINIT_P): Check DECL_NOINIT_P before using
	unnamed bss/lcomm sections for bss_initializer variables.
	(default_elf_select_section): Use DECL_NOINIT_P instead of
	looking up attribute for .noinit section selection.
	(default_unique_section): Check DECL_NOINIT_P for .noinit
	section selection.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/execute/noinit-attribute.c: Don't override
	optimization options set by torture test harness.
	* lib/target-supports.exp (check_effective_target_noinit): Adjust
	comment formatting.