pico_setup.sh doesn't install all of the required components on an AARCH64 kernel/32-bit userland.

Open pico_setup.sh and inspect the package installation list, then compare it with the two missing packages identified in the issue: libnewlib-arm-none-eabi and libstdc++-arm-none-eabi-newlib. Done means the setup script installs both packages and the reported AARCH64-kernel, 32-bit-userland build no longer fails with the missing nosys.specs error.

Running pico_setup.sh on a RaspiOS system built as 32-bit userland but running with the 64-bit 5.10.10 kernel fails with

Scanning dependencies of target ELF2UF2Build
Scanning dependencies of target bs2_default
[  0%] Creating directories for 'ELF2UF2Build'
[  0%] Building ASM object pico_sdk/src/rp2_common/boot_stage2/CMakeFiles/bs2_default.dir/boot2_w25q080.S.obj
[  0%] Linking ASM executable bs2_default.elf
[  0%] No download step for 'ELF2UF2Build'
arm-none-eabi-gcc: error: nosys.specs: No such file or directory
make[2]: *** [pico_sdk/src/rp2_common/boot_stage2/CMakeFiles/bs2_default.dir/build.make:77: pico_sdk/src/rp2_common/boot_stage2/bs2_default.elf] Error 1
make[1]: *** [CMakeFiles/Makefile2:1016: pico_sdk/src/rp2_common/boot_stage2/CMakeFiles/bs2_default.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
[  0%] No update step for 'ELF2UF2Build'
[  0%] No patch step for 'ELF2UF2Build'
[  0%] Performing configure step for 'ELF2UF2Build'
-- The C compiler identification is GNU 8.3.0
-- The CXX compiler identification is GNU 8.3.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /shared/pico-setup/pico/pico-examples/build/elf2uf2
[  0%] Performing build step for 'ELF2UF2Build'
Scanning dependencies of target elf2uf2
[ 50%] Building CXX object CMakeFiles/elf2uf2.dir/main.cpp.o
[100%] Linking CXX executable elf2uf2
[100%] Built target elf2uf2
[  0%] No install step for 'ELF2UF2Build'
[  0%] Completed 'ELF2UF2Build'
[  0%] Built target ELF2UF2Build
make: *** [Makefile:84: all] Error 2

The solution is to install libnewlib-arm-none-eabi and libstdc++-arm-none-eabi-newlib.

pico-setup.sh needs to be updated to include those two pieces.