Teach cvmach to emit the symbol table.

Teach cvmach to emit the symbol table.

This preserves the name and value of every symbol.  The type and other
info of a symbol might be lost.  In gdb, one can now "disas main" or
"disas '.ret'" to disassemble functions by name.

Most symbols are in sections, so I also teach cvmach to emit the Mach
section headers.  The entry point in plat/osx*/descr moves down to
make room for the section headers and LC_SYMTAB.

I fix some bugs in calculations of cvmach.  They were wrong if ROM had
a greater alignment than TEXT, or if DATA did not start on a page
boundary.  I introduce machseg[] to simplify the mess of variables in
main().  I declare most functions as static.  Also, cvmach becomes the
first program to #include <object.h>.