Trying to get LLVM libc to build in pkgsLLVM.pkgsLLVMLibc.stdenv, aside from dropping compiler-rt and libunwind because those fail, this is also a problem:

FAILED: libc/src/math/generic/CMakeFiles/libc.src.math.generic.issignalingl.dir/issignalingl.cpp.o
/nix/store/rsdq8qwhg80fnmkv1frwdplzqjbrlx68-aarch64-unknown-linux-llvm-clang-wrapper-21.0.0-unstable-2025-03-16/bin/aarch64-unknown-linux-llvm-clang++ -DLIBC_NAMESPACE=__llvm_
libc_21_0_0_git -I/build/libc-src-21.0.0-unstable-2025-03-16/libc -isystem /build/libc-src-21.0.0-unstable-2025-03-16/runtimes/build/libc/include -Werror=date-time -Werror=ung
uarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexc
ept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wno-comment -Wstring-conversion -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=gnu++17 -DLIBC
_QSORT_IMPL=LIBC_QSORT_QUICK_SORT -DLIBC_ADD_NULL_CHECKS -fpie -ffreestanding -DLIBC_FULL_BUILD -nostdlibinc -idirafter=/usr/include -ffixed-point -fno-builtin -fno-exceptions
 -fno-lax-vector-conversions -fno-unwind-tables -fno-asynchronous-unwind-tables -fno-rtti -ftrivial-auto-var-init=pattern -fno-omit-frame-pointer -Wall -Wextra -Werror -Wconve
rsion -Wno-sign-conversion -Wdeprecated -Wno-c99-extensions -Wno-gnu-imaginary-constant -Wno-pedantic -Wimplicit-fallthrough -Wwrite-strings -Wextra-semi -Wnewline-eof -Wnonpo
rtable-system-include-path -Wstrict-prototypes -Wthread-safety -Wglobal-constructors -DLIBC_COPT_PUBLIC_PACKAGING -MD -MT libc/src/math/generic/CMakeFiles/libc.src.math.generi
c.issignalingl.dir/issignalingl.cpp.o -MF libc/src/math/generic/CMakeFiles/libc.src.math.generic.issignalingl.dir/issignalingl.cpp.o.d -o libc/src/math/generic/CMakeFiles/libc
.src.math.generic.issignalingl.dir/issignalingl.cpp.o -c /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/math/generic/issignalingl.cpp
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/math/generic/issignalingl.cpp:10:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/FPUtil/BasicOperations.h:12:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/FPUtil/FEnvImpl.h:24:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/FPUtil/aarch64/FEnvImpl.h:25:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/FPUtil/FPBits.h:21:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/libc_assert.h:25:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/OSUtil/io.h:19:
/build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/OSUtil/linux/io.h:16:10: fatal error: 'sys/syscall.h' file not found
   16 | #include <sys/syscall.h> // For syscall numbers.
      |          ^~~~~~~~~~~~~~~

I checked the linux headers derivation being used and it doesn't have a sys dir in the include dir.