Major 7ths broken in 2.3.x Jazz notation

[Matthias Neeracher]

I recently noticed that for the past few releases, the hollow triangle 
symbol for :maj7 is printed as a (raised) M.
This is evident, for example, in the chord-names-jazz.ly file. I 
suspect that it has something to do with the major changes in font 
handling in 2.3, as the triangle symbol seems to be in the "M" position 
in the TeX math font.

Matthias