[AVR] Use the correct address space for non-prototyped function calls
Some function declarations like this:
do not have a type declaration, for that you'd use:
Clang internally bitcasts the variadic function declaration to a
function pointer, but doesn't use the correct address space on AVR. This
commit fixes that.
This fix is necessary to let Clang compile compiler-rt for AVR.
Differential Revision: https://reviews.llvm.org/D78125