feat(codegen): production executable format — PIE, DWARF, libc linkage, object output

Problem

Native binaries are not production-grade: Linux output is a fixed-address ET_EXEC (no ASLR); x86_64 Mach-O sets only MH_NOUNDEFS (no MH_PIE); no DWARF is written anywhere so debuggers can't map to source; macOS output makes raw syscalls, whose ABI Apple does not guarantee; and there is no .o output for linking with C.

Evidence (origin/main @ d4b219507)

• packages/codegen/src/elf.zig:76
• packages/codegen/src/aarch64_native_codegen.zig:244
• packages/codegen/src/macho.zig:208-209
• packages/codegen/src/native_codegen.zig:4687-4714

Scope

• Emit position-independent code and ET_DYN / MH_PIE executables.
• Emit DWARF line tables (at minimum) and symbols.
• Link libSystem on macOS instead of raw syscalls; optional libc on Linux.
• --emit=obj to produce relocatable objects.

Acceptance criteria

• readelf -h shows DYN; checksec reports PIE on Linux and macOS.
• lldb/gdb can break on a Home source line.
• An --emit=obj object links with cc into a working program.

Links

Related: #15 Part of #814.