ld: Load the object only section when opening the mixed object file

Load the object only section when opening the mixed object file, instead
of loading it after all other input files have been loaded. This fixed

.../ld/collect-ld: /tmp/ccZAoUIW.obj-only.o: in function `main':
.../ld/testsuite/ld-plugin/lto-10a.c:4: multiple definition of `main'; /usr/x86_64-w64-mingw32/sys-root/mingw/lib/../lib/libmingw32.a(lib64_libmingw32_a-crtexewin.o):(.text.startup+0x0): first defined here
.../ld/collect-ld: /usr/x86_64-w64-mingw32/sys-root/mingw/lib/../lib/libmingw32.a(lib64_libmingw32_a-crtexewin.o):(.text.startup+0xc5): undefined reference to `WinMain'
collect2: error: ld returned 1 exit status
...
FAIL: LTO 10

for x86_64-w64-mingw32 so that mixing LTO and non-LTO relocatable files
for "ld -r" works for both ELF and non-ELF platforms.

	* ld.texi: Remove "On ELF platforms" from documentation of mixing
	LTO and non-LTO relocatable files for "ld -r".
	* ldlang.c (cmdline_load_object_only_section): New.
	(cmdline_check_object_only_section): Call it.
	* testsuite/ld-plugin/lto.exp: Enable mixed LTO and non-LTO
	relocatable output tests for all.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>