Improvements in amd64

[Maxime Villard <max%m00nbsd.net@localhost>]

I've committed several improvements in amd64 these last days.

In chronological order, for the record:
 - I cleaned up the asm code and fixed several comments, which makes the
   boot process much easier to understand.
 - I fixed the alignment for the text segment, so that it can be covered by
   more large pages [1] - thereby reducing TLB contention.
 - I fixed a bug in the way the secondary CPUs are launched [2], which
   caused them to crash if they tried to access an X-less page.
 - I took rodata out of the text+rodata chunk, and put it in the data+bss+
   PRELOADED_MODULES+BOOTSTRAP_TABLES chunk [3]. rodata was no longer large
   page optimized, and had RWX permissions.
 - I retook rodata out of the rodata+data+bss+PRELOADED_MODULES+
   BOOTSTRAP_TABLES chunk, and made the kernel map it independently without
   the W permision [4].
 - I made the kernel map rodata without the X permission, by using the NOX
   bit on its pages [5] (now that the secondary CPUs could handle that
   properly).
 - I took the data+bss chunk out of the data+bss+PRELOADED_MODULES+
   BOOTSTRAP_TABLES chunk, and made the kernel map it independently without
   X permission [6].
 - I made the kernel remap rodata and data+bss with large pages and proper
   permissions [7] - which reduces once again TLB contention.

Now, the way the kernel image is mapped is more flexible, more secure and
more performant. There are still several things to fix, and the same
procedure needs to be applied in i386.

If you have questions, comments, if you think something is wrong, or if you
spot any mistakes, feel free to tell me about it. Also, if someone could
review the last change, that would be nice.

[1] http://mail-index.netbsd.org/source-changes/2016/05/07/msg074501.html
[2] http://mail-index.netbsd.org/source-changes/2016/05/11/msg074613.html
[3] http://mail-index.netbsd.org/source-changes/2016/05/12/msg074625.html
[4] http://mail-index.netbsd.org/source-changes/2016/05/12/msg074627.html
[5] http://mail-index.netbsd.org/source-changes/2016/05/12/msg074628.html
[6] http://mail-index.netbsd.org/source-changes/2016/05/12/msg074632.html
[7] http://mail-index.netbsd.org/source-changes/2016/05/13/msg074650.html