fix: build system cleanup — replace non-freestanding <string.h> with utils.h, paramet...

fix: build system cleanup — replace non-freestanding <string.h> with utils.h, parameterize toolchain, fix header conflicts

- Kernel: replace #include <string.h> (not freestanding) with #include "utils.h"
  in src/arch/x86/fpu.c, src/kernel/fpu.c, src/rump/rumpuser_adros.c
- Makefile: define USER_CC/USER_LD/USER_AR variables for userspace cross-compilation,
  replacing all hardcoded i686-elf-gcc/ld references in FULLTEST, DYN_CC, DYN_LD,
  LDSO, PIE_SO, PIE_ELF rules
- Makefile: pass USER_CC/USER_LD/USER_AR to ulibc sub-make for consistent toolchain
- Makefile: add missing .PHONY targets (test-battery, run-arm, run-riscv, run-mips)
- ulibc Makefile: use ?= for CC/AS/AR/LD so parent can override
- doom Makefile: use ?= for CC/LD so parent can override
- ulibc: remove duplicate stat/fstat declarations from unistd.h (conflicted with
  sys/stat.h when both included); proper declarations remain in sys/stat.h per POSIX