[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/boot/allocator.cpp

@@ -155,7 +155,7 @@ allocator::memset(void *start, size_t size, uint8_t value)
 }
 
 void
-allocator::copy(void *to, void *from, size_t size)
+allocator::copy(void *to, const void *from, size_t size)
 {
     m_bs.copy_mem(to, from, size);
 }