fix: doom build + move userspace build output to build/$ARCH/user/

fix: doom build + move userspace build output to build/$ARCH/user/

- Fix doom.elf build: force i686-elf toolchain (CC/LD := instead of ?=),
  add --unresolved-symbols=ignore-in-shared-libs for libc.so undefined refs
- Reorganize userspace build output from build/user/ to build/$ARCH/user/
  (e.g. build/x86/user/) so all arch artifacts live under one directory
- Sub-Makefiles (common.mk, fulltest, ldso, pie_test) now accept BUILDDIR
  override from root Makefile via ?= operator
- Root Makefile passes BUILDDIR=$(CURDIR)/$(USER_BUILD)/cmds/<name> to
  all sub-Make invocations

Tests: 103/103 smoke, 68/68 host utils, cppcheck clean