Nowadays, Flash is cheaper than ROM for all but the smallest microcontrollers (like 8 bit
size). GlibC is huge, so it doesn't make sense to put it in ROM. If it's in flash, they're
going to leave themselves a way to write it (even if it's just thru JTAG). Therefore it's
unlikely anyone distributes a completely non-writable GlibC (or even Linux kernel for that
matter).