How to make char unsigned by default in target?

How can I make char unsigned by default in a target? I want to make my
target to behave by default as if -fno-signed-char had been specified.

TargetInfo has lots of members to specify properties of types, e.g.
WCharType (which can be overridden by options, such as
-fno-short-wchar). But I didn't see a way to specify that char should be
unsigned.

Philipp