[kernel] Hide kernel symbols by default

Using `-fvisibility=hidden` when building the kernel, and then
`--discard-all` when stripping it, we shave almost 100KiB off of the
resulting ELF file.

Also dropped some unused symbols from the linker script, and rearranged
the sections so that the file is able to be mapped directly into memory
instead of having each section copied.

src/libraries/elf/elf/file.h

@@ -22,7 +22,7 @@ public:
     inline size_t size() const { return m_size; }
     inline unsigned count() const { return m_count; }
 
-    inline const T & operator [] (int i) const { return *util::offset_pointer<T>(m_start, m_size*i); }
+    inline const T & operator [] (int i) const { return *util::offset_pointer<const T>(m_start, m_size*i); }
     inline const iterator begin() const { return iterator(m_start, m_size); }
     inline const iterator end() const { return util::offset_pointer(m_start, m_size*m_count); }