How to build the FreeBSD bootloader?

[probonopd]

cd /tmp
sudo rm -rf /usr/src
fetch ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/amd64/13.1-RELEASE/src.txz
sudo tar -C / -xvf src.txz

cd /usr/src/stand
sudo make

ends with

===> userboot (all)
===> userboot/test (all)
===> userboot/userboot_4th (all)
building shared library userboot_4th.so
cc  -nostdlib -Wl,-Bsymbolic    -shared -Wl,-x -Wl,--fatal-warnings -Wl,--warn-shared-textrel  -o userboot_4th.so.full -Wl,-soname,userboot_4th.so autoload.pico bcache.pico biossmap.pico bootinfo.pico bootinfo32.pico bootinfo64.pico conf.pico copy.pico devicename.pico elf32_freebsd.pico elf64_freebsd.pico host.pico main.pico userboot_cons.pico userboot_disk.pico vers.pico gfx_fb.pico boot.pico commands.pico console.pico devopen.pico interp.pico interp_backslash.pico interp_parse.pico ls.pico misc.pico module.pico nvstore.pico pnglite.pico tslog.pico load_elf32.pico load_elf32_obj.pico reloc_elf32.pico load_elf64.pico load_elf64_obj.pico reloc_elf64.pico disk.pico part.pico vdisk.pico interp_forth.pico zfs_cmd.pico /usr/obj/usr/src/amd64.amd64/stand/ficl/libficl.a /usr/obj/usr/src/amd64.amd64/stand/libsa/libsa.a 
/usr/local/bin/x86_64-unknown-freebsd13.1-ld: warning: _setjmp.o: missing .note.GNU-stack section implies executable stack
/usr/local/bin/x86_64-unknown-freebsd13.1-ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
cc: error: linker command failed with exit code 1 (use -v to see invocation)
*** Error code 1

Stop.
make[2]: stopped in /usr/src/stand/userboot/userboot_4th

sudo make -n

ends with

===> i386/boot2 (all)
case machine in  machine)  if [ 1 -eq 0 ]; then  path=/usr/src/sys/amd64/include ;  else  path=/usr/src/sys/i386/include ;  fi ;;  *)  path=/usr/src/sys/machine/include ;;  esac ;  case include/machine in  */*) mkdir -p include;;  esac ;  path=`(cd $path && /bin/pwd)` ;  echo include/machine "->" $path ;  ln -fns $path include/machine
case x86 in  machine)  if [ 1 -eq 0 ]; then  path=/usr/src/sys/amd64/include ;  else  path=/usr/src/sys/i386/include ;  fi ;;  *)  path=/usr/src/sys/x86/include ;;  esac ;  case include/x86 in  */*) mkdir -p include;;  esac ;  path=`(cd $path && /bin/pwd)` ;  echo include/x86 "->" $path ;  ln -fns $path include/x86
cc -O2 -pipe -fno-common   -I/usr/src/stand/i386/btx/lib -nostdinc    -I/usr/obj/usr/src/amd64.amd64/stand/libsa32 -I/usr/src/stand/libsa -D_STANDALONE -I/usr/src/sys -Ddouble=jagged-little-pill -Dfloat=floaty-mcfloatface -ffunction-sections -fdata-sections -DLOADER_GELI_SUPPORT -I/usr/src/stand/libsa/geli -DLOADER_DISK_SUPPORT -m32 -ffreestanding -mno-mmx -mno-sse -mno-avx -mno-avx2 -msoft-float -march=i386 -mno-relax -I. -Iinclude -fomit-frame-pointer  -mrtd  -mregparm=3  -DUFS1_AND_UFS2  -DFLAGS=0x80  -DSIOPRT=0x3f8  -DSIOFMT=0x3  -DSIOSPD=9600  -I/usr/src/stand/common  -Wall -Waggregate-return -Wbad-function-cast -Wno-cast-align  -Wmissing-declarations -Wmissing-prototypes -Wnested-externs  -Wpointer-arith -Wshadow -Wstrict-prototypes -Wwrite-strings -g  -std=gnu99 -Wno-format-zero-length -Wsystem-headers -Werror -Wno-pointer-sign -Wno-empty-body -Wno-string-plus-int -Wno-unused-const-variable -Wno-error=unused-but-set-variable -Wno-tautological-compare -Wno-unused-value -Wno-parentheses-equality -Wno-unused-function -Wno-enum-conversion -Wno-unused-local-typedef -Wno-address-of-packed-member -Wno-switch -Wno-switch-enum -Wno-knr-promoted-parameter -Wno-parentheses  -Oz -Oz -mstack-alignment=8 -mllvm -inline-threshold=3 -mllvm -enable-load-pre=false -Qunused-arguments       -c boot1.S -o boot1.o
ld -m elf_i386_fbsd -static -N --gc-sections -e start --defsym ORG=0x7c00 -T /usr/src/stand/i386/boot.ldscript -o boot1.out boot1.o
nm -t d boot1.out | awk '/([0-9])+ T xread/  { x = $1 - ORG1;  printf("#define XREADORG %#x\n", REL1 + x) }'  ORG1=`printf "%d" 0x7c00`  REL1=`printf "%d" 0x700` > boot2.h
objcopy -S -O binary boot1.out boot1
dd status=none if=/dev/zero of=boot2.ldr bs=512 count=1
make[2]: don't know how to make /usr/obj/usr/src/amd64.amd64/stand/i386/btx/lib/crt0.o. Stop

make[2]: stopped in /usr/src/stand/i386/boot2
*** Error code 2

Stop.
make[1]: stopped in /usr/src/stand/i386
*** Error code 1

Stop.
make: stopped in /usr/src/stand