libstdc++: Allow new-abi-baseline target to overwrite existing file

libstdc++: Allow new-abi-baseline target to overwrite existing file

There doesn't seem to be much benefit to writing the symbols to
baseline_symbols.txt.new when an existing file is already present. It
just adds a manual step for maintainers to move the .txt.new file to
replace the .txt one. Overwriting the file directly allows you to use
git diff to see what changed immediately, and you can easly use git
commands to revert to the original file too.

libstdc++-v3/ChangeLog:

	* testsuite/Makefile.am (new-abi-baseline): Overwrite existing
	file instead of creating baseline_symbols.txt.new.
	* testsuite/Makefile.in: Regenerate.