lto: Remove link() to fix build with MinGW [PR118238]

I used link() to create cheap copies of Incremental LTO cache contents
to prevent their deletion once linking is finished.
This is unnecessary, since output_files are deleted in our lto-plugin
and not in the linker itself.

Bootstrapped/regtested on x86_64-linux.
lto-wrapper now again builds on MinGW. Though so far I have not setup
MinGW to be able to do full bootstrap.
Ok for trunk?

	PR lto/118238

gcc/ChangeLog:

	* lto-wrapper.cc (run_gcc): Remove link() copying.

lto-plugin/ChangeLog:

	* lto-plugin.c (cleanup_handler):
	Keep output_files when using Incremental LTO.
	(onload): Detect Incremental LTO.