It's up to the individual merge driver. From man gitattributes(5):
There are a few built-in low-level merge drivers defined that can be
asked for via the merge attribute.
text
Usual 3-way file level merge for text files. Conflicted regions
are marked with conflict markers <<<<<<<, ======= and >>>>>>>. The
version from your branch appears before the ======= marker, and the
version from the merged branch appears after the ======= marker.
[…]
Defining a custom merge driver
The definition of a merge driver is done in the .git/config file, not
in the gitattributes file, so strictly speaking this manual page is a
wrong place to talk about it. However…
To define a custom merge driver filfre, add a section to your
$GIT_DIR/config file (or $HOME/.gitconfig file) like this:
[merge "filfre"]
name = feel-free merge driver
driver = filfre %O %A %B
recursive = binary
That is: the "text" merge driver does what you don't like, and cannot be configured as to what symbol to use (though you can configure it to use a bigger number of markers). But you can copy-paste the driver, rename it, replace the symbol with yours, and then configure git to use your driver.