I finally found the solution (or, alternative, I suppose). Apparently, the && operator can be used to get the address of C labels, removing the need for me to use inline assembly at all. I don't think it's in the C standard, but it looks like Clang supports it, and I've heard GCC does too.
#include <stdio.h>
int main(void){
foo_start:
printf("Hello, world.\n");
foo_end:
printf("Foo has ended.");
void* foo_start_ptr = &&foo_start;
void* foo_end_ptr = &&foo_end;
printf("foo_start: %p, foo_end: %p\n", foo_start_ptr, foo_end_ptr);
printf("Difference: 0x%tx\n", (long)foo_end_ptr - (long)foo_start_ptr);
return 0;
}
Now, this only works if the labels are in the same function, but for what I intend to use this for, it's perfect. No more ASM, and it doesn't leave a symbol behind. It appears to work just how I need it to. (Not tested on ARM64)