[ld.so] Add a minimal dynamic linker

This commit includes a number of changes to enable loading of PIE
executables:

- The loader in srv.init checks for a `PT_INTERP` segment in the program
  its loading, and if it exists, loads the specified interpreter and
  passes control to it instead of the program itself.
- Added ld.so the dynamic linker executable and set it as the
  interpreter for all user-target programs.
- Program initial stack changed again to now contain a number of
  possible tagged structures, including a new one for ld.so's arguments,
  and for passing handles tagged with protocol ids.
- Added a stub for a new VFS protocol. Unused so far, but srv.init will
  need to serve VFS requests from ld.so once I transition libraries to
  shared libs for user-target programs. (Right now all executables are
  PIE but statically linked, so they only need internal relocations.)
- Added 16 and 8 bit variants of `util::bitset`. This ended up not being
  used, but could be useful.

src/user/ld.so/start.s

@@ -0,0 +1,33 @@
+extern ldso_init
+extern _GLOBAL_OFFSET_TABLE_
+
+global _ldso_start:function hidden (_ldso_start.end - _ldso_start)
+_ldso_start:
+    mov rbp, rsp
+
+    ; Save off anything that might be a function arg
+    push rdi
+    push rsi
+    push rdx
+    push rcx
+    push r8
+    push r9
+
+    ; Call ldso_init with the loader-provided stack data and
+    ; also the address of the GOT, since clang refuses to take
+    ; the address of it, only dereference it.
+    mov rdi, rbp
+    lea rsi, [rel _GLOBAL_OFFSET_TABLE_]
+    call ldso_init
+    ; The real program's entrypoint is now in rax
+
+    ; Put the function call params back
+    pop r9
+    pop r8
+    pop rcx
+    pop rdx
+    pop rsi
+    pop rdi
+
+    jmp rax
+.end: