Before this, you could make the kernel copy memory from anywhere by
setting up an ELF executable with a program header specifying file
offsets outside the file.
Since ELFImage didn't even know how large it was, we had no clue that
we were copying things from outside the ELF.
Fix this by adding a size field to ELFImage and validating program
header ranges before memcpy()'ing to them.
The ELF code is definitely going to need more validation and checking.
Turns out we can use abi::__cxa_demangle() for this, and all we need to
provide is sprintf(), realloc() and free(), so this patch exposes them.
We now have fully demangled C++ backtraces :^)
