[AVR] Do not place functions in .progmem.data

Previously, the AVR backend would put functions in .progmem.data. This
is probably a regression from when functions still lived in address
space 0. With this change, only global constants are placed in
.progmem.data.

This is not complete: avr-gcc additionally respects -fdata-sections for
progmem global constants, which LLVM doesn't yet do. But fixing that is
a bit more complicated (and I believe other backends such as RISC-V
might also have similar issues).

Differential Revision: https://reviews.llvm.org/D78212