[kernel] Split loading from scheduler

In preparation for moving things to the init process, move process
loading out of the scheduler. memory_bootstrap now has a
load_simple_process function for mapping an args::program into memory,
and the stack setup has been simplified (though all the initv values are
still being added by the kernel - this needs rework) and normalized to
use the thread::add_thunk_user code path.

modules.yaml

@@ -29,7 +29,6 @@ modules:
             - src/kernel/interrupts.cpp
             - src/kernel/interrupts.s
             - src/kernel/io.cpp
-            - src/kernel/loader.s
             - src/kernel/log.cpp
             - src/kernel/main.cpp
             - src/kernel/memory_bootstrap.cpp
@@ -314,6 +313,7 @@ modules:
             - src/tests/main.cpp
             - src/tests/map.cpp
             - src/tests/vector.cpp
+
 overlays:
     - url: https://f000.backblazeb2.com/file/jsix-os/sysroot-llvm8-20190706.tar.bz2
       path: sysroot