Oh, my bad: this second issue is actually fixed by adding a -L pointing to the same directory so that -lgcc is interpreted the right way by the linker:
[andrew@baltar ~]$ /data/andrew/llvm-root/bin/clang -v --target=aarch64-linux-gnu --sysroot=/usr/aarch64-linux-gnu -B/usr/lib/gcc/aarch64-linux-gnu/10.2.0/ -L/usr/lib/gcc/aarch64-linux-gnu/10.2.0/ test.c -o test
clang version 11.0.0 (https://github.com/llvm/llvm-project.git 176249bd6732a8044d457092ed932768724a6f06)
Target: aarch64-unknown-linux-gnu
Thread model: posix
InstalledDir: /data/andrew/llvm-root/bin
"/data/andrew/llvm-root/bin/clang-11" -cc1 -triple aarch64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name test.c -mrelocation-model static -mframe-pointer=non-leaf -fmath-errno -fno-rounding-math -mconstructor-aliases -target-cpu generic -target-feature +neon -target-abi aapcs -fallow-half-arguments-and-returns -fno-split-dwarf-inlining -debugger-tuning=gdb -v -resource-dir /data/andrew/llvm-root/lib/clang/11.0.0 -isysroot /usr/aarch64-linux-gnu -internal-isystem /usr/aarch64-linux-gnu/usr/local/include -internal-isystem /data/andrew/llvm-root/lib/clang/11.0.0/include -internal-externc-isystem /usr/aarch64-linux-gnu/include -internal-externc-isystem /usr/aarch64-linux-gnu/usr/include -fdebug-compilation-dir /home/andrew -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fcolor-diagnostics -faddrsig -o /tmp/test-26b9f4.o -x c test.c
clang -cc1 version 11.0.0 based upon LLVM 11.0.0 default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/usr/aarch64-linux-gnu/usr/local/include"
ignoring nonexistent directory "/usr/aarch64-linux-gnu/usr/include"
#include "..." search starts here:
#include <...> search starts here:
/data/andrew/llvm-root/lib/clang/11.0.0/include
/usr/aarch64-linux-gnu/include
End of search list.
"/usr/bin/aarch64-linux-gnu-ld" --sysroot=/usr/aarch64-linux-gnu -EL --eh-frame-hdr -m aarch64linux -dynamic-linker /lib/ld-linux-aarch64.so.1 -o test /usr/aarch64-linux-gnu/lib/crt1.o /usr/aarch64-linux-gnu/lib/crti.o /usr/lib/gcc/aarch64-linux-gnu/10.2.0/crtbegin.o -L/usr/lib/gcc/aarch64-linux-gnu/10.2.0/ -L/usr/aarch64-linux-gnu/lib/../lib64 -L/usr/aarch64-linux-gnu/lib /tmp/test-26b9f4.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/aarch64-linux-gnu/10.2.0/crtend.o /usr/aarch64-linux-gnu/lib/crtn.o
Problem solved! It feels a little awkward to have to pass -B/path -L/path but everything works now. Thanks again Mehdi for your help!