[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/loader.h

@@ -28,14 +28,16 @@ load_file(
     const wchar_t *path);
 
 /// Parse and load an ELF file in memory into a loaded image.
-/// \arg disk       The opened UEFI filesystem to load from
-/// \arg desc       The descriptor identifying the program
-/// \arg name       The human-readable name of the program to load
+/// \arg disk   The opened UEFI filesystem to load from
+/// \arg desc   The descriptor identifying the program
+/// \arg name   The human-readable name of the program to load
+/// \arg verify If this is the kernel and should have its header verified
 bootproto::program *
 load_program(
     fs::file &disk,
     const wchar_t *name,
-    const descriptor &desc);
+    const descriptor &desc,
+    bool verify = false);
 
 /// Load a file from disk into memory, creating an init args module
 /// \arg disk    The opened UEFI filesystem to load from
@@ -51,10 +53,5 @@ load_module(
     bootproto::module_type type,
     uint16_t subtype);
 
-/// Verify that a loaded ELF has the j6 kernel header
-/// \arg program  The program to check for a header
-void
-verify_kernel_header(bootproto::program &program);
-
 } // namespace loader
 } // namespace boot