Update LTO plugin interface

[H.J. Lu]

Hi,

Here is a proposal to update LTO plugin interface.  Any comments?

Thanks.

-- 
H.J.
---
Goal:  We should preserve the same linker command line order as if
there are no IR.
Problem:
	a. LTO may generate extra symbol references which aren't in IR.
	b. It was worked around with -pass-through hack.  But it doesn't
preserve the link command line order.

Proposal:
	a. Remove -pass-through hack in GCC.
	b. Compiler plugin controls what linker uses to generate the final executable:
		i. The linker command line order should be the same, with or without LTO.
	c. Add a cmdline bit field to
	struct ld_plugin_input_file
	{
	   const char *name;
	   int fd;
	   off_t offset;
	   off_t filesize;
	   void *handle;
	   unsigned int cmdline : 1;
	};
	It is used by linker to tell plugin that the input file comes from
linker command line.
	d. 2 stage linker:
		i. Stage 1: Normal symbol resolution with plugin.
		ii. Stage 2:
			1) Call the "all symbols read" handler to get the final linker inputs.
			2) Discard all previous inputs.
			3) Generate the final executable with inputs from plugin.
	e. Compiler plugin:
		i. For a file, which comes from the linker command line and isn't
claimed by plugin, save it in the linker pass-through list in the same
order as it comes in.
		ii. For the first file claimed by plugin,  remember the last
pass-through linker input.
		iii. The "all symbols read" handler adds input files to the linker
in the order:
			1) All linker input files on the linker pass-through list up to the
first file claimed by plugin.
			2) All linker input files generated by plugin.
			3) The rest of linker input files on the linker pass-through list.
	f. Limitation:
		i. All files claimed by plugin are grouped together.  Any archives
between files claimed by plugin are placed after all linker input
files generated by plugin when passed to linker.