toolchain: fix libgloss/adros build errors

toolchain: fix libgloss/adros build errors

- Add sys/uio.h compat header (struct iovec + readv/writev declarations)
  needed by posix_stubs.c which was missing from sysroot_headers
- Remove duplicate fcntl() definition in posix_stubs.c (line 541 was
  identical to line 256, causing redefinition error with newlib headers)
- Move libgloss/adros source sync outside patch_newlib() marker guard
  so edits to stubs are always picked up on rebuild (cp -f instead of
  cp -u); keep mkdir -p inside patch_newlib() for autoconf file creation