Support for Win32 safeseh (BR 195417)

Support for Win32 safeseh (BR 195417)

This is basically not a bug report, but a feature request.

It's desired to be able to link .obj modules compiled with 'nasm -f
win32' with Microsoft 'link /safeseh'. As well as to register symbols
(commonly subroutine's entry points or even external symbols) as "safe
handlers." In order to achieve this, several points are required.

First of all, object module has to have absolute symbol named @feat.00
with value of 1. This can actually be achived by adding 'absolute 1'
and '@feat.00:' to source code, but it's desirable that it's
autogenerated for win32 modules.

Handler registration is essentially symbol's *index* in current
module's symbol table in .sxdata, segment with 0x200 segment flags, an
"info" segment. It's also essential that symbol has type 0x20 (see
below). All this is depicted in following framgents of 'objdump -xD'
output:

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .sxdata       00000004  00000000  00000000  0000003c  2**2
                  CONTENTS, READONLY, DEBUGGING

SYMBOL TABLE:
...
[  5](sec -1)(fl 0x00)(ty   0)(scl   3) (nx 0) 0x00000001 @feat.00
[  6](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _handler
...
<.sxdata>
0: 06 00 00 00

Note [6] and (ty 20) in _handle line in SYMBOL TABLE. "06 00 00 00" in
.sxdata is little-endian 6, _handler's index. This is what makes up
"registration." It's impossible to achieve this with current nasm
facilities and it's probably appropriate to introduce a directive for
it, 'safeseh _handler' is probably most natural choice.