Kernel+LibELF: Don't blindly trust ELF symbol offsets in symbolication

It was possible to craft a custom ELF executable that when symbolicated
would cause the kernel to read from user-controlled addresses anywhere
in memory. You could then fetch this memory via /proc/PID/stack

We fix this by making ELFImage hand out StringView rather than raw
const char* for symbol names. In case a symbol offset is outside the
ELF image, you get a null StringView. :^)

Test: Kernel/elf-symbolication-kernel-read-exploit.cpp

Kernel/KSyms.h

@@ -8,7 +8,7 @@ struct KSym {
     const char* name;
 };
 
-u32 address_for_kernel_symbol(const char* name);
+u32 address_for_kernel_symbol(const StringView& name);
 const KSym* ksymbolicate(u32 address);
 void load_ksyms();