add code to emit the .text section to the section header.

Add a *VERY INITIAL* machine code emitter class.  This is enough to take
this C function:
int foo(int X) { return X +1; }

and make objdump produce the following:

$ objdump -d t-llvm.o

t-llvm.o:     file format elf32-i386

Disassembly of section .text:

00000000 <.text>:
   0:   b8 01 00 00 00          mov    $0x1,%eax
   5:   03 44 24 04             add    0x4(%esp,1),%eax
   9:   c3                      ret


Anything using branches or refering to the constant pool or requiring
relocations will not work yet.

llvm-svn: 22375