Global vs. system-wide settings
There is some ambiguity in your question's terminology.
In a Git context, "global" usually means "user-level"; in other words, a global setting affect all repositories for one specific user (the active one). In contrast, a system-wide setting affects all repositories for all users of a machine.
Repository-level gitattributes
(I'm only mentioning this for completeness.)
According to the relevant section of the Pro Git book,
If you wish to affect only a single repository (i.e., to assign attributes to files that are particular to one user’s workflow for that repository), then attributes should be placed in the $GIT_DIR/info/attributes file.
$GIT_DIR would typically expand to <path-to-repo-root-directory>/.git.
Global (user-level) gitattributes
According to the relevant section of the Pro Git book,
Attributes that should affect all repositories for a single user should be placed in a file specified by the core.attributesfile configuration option [...]. Its default value is $XDG_CONFIG_HOME/git/attributes. If $XDG_CONFIG_HOME is either not set or empty, $HOME/.config/git/attributes is used instead.
You can also run the following command,
git config --global core.attributesfile <path>
to point Git to a custom path <path> for your global gitattributes file, e.g. ~/.gitattributes.
System-wide gitattributes
According to the relevant section of the Pro Git book,
Attributes for all users on a system should be placed in the $(prefix)/etc/gitattributes file.
which naturally begs the question:
[...] But where is $(prefix)?
See What is $(prefix) on $(prefix)/etc/gitconfig? for an answer. Unless you've assigned prefix a custom, non-empty value, $(prefix) expands to nothing by default; therefore, your system-wide gitattributes file should reside in /etc/.
