ld: Fix LTO for MinGW targets

ld: Fix LTO for MinGW targets

When creating a dummy BFD for an IR file, the output BFD is used as
a template for the new BFD, when it needs to be the input BFD passed
into the function when not dealing with a BFD plugin.

On most targets this is not an issue as the input and output formats
are the same anyway, but on MinGW targets, there are two variant
formats used (pe-i386/pe-x86-64 and pei-i386/pei-x86-64) which are
similar but not interchangeable here.

	PR ld/18199
	* plugin.c (plugin_get_ir_dummy_bfd): Use srctemplate as the
	template when calling bfd_create if it does not use the BFD
	plugin target vector.