When the path is already absolute, the call to current_path() is
wasteful, because operator/ will ignore the left operand anyway.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/99876
	* src/c++17/fs_ops.cc (fs::absolute): Call non-throwing form,
	to avoid unnecessary current_path() call.

Tested powerpc64le-linux. Committed to trunk.

I'll backport to gcc-10 and gcc-11 too.