The Linux kernel prefers GAS (or clang) w/ the C preprocessor. Luckily most of clang's assembler unit tests (and LLVM's MC layer unit tests) also use GAS style syntax.
GNU Assembler (GAS) ≥2.45 on x86-64: call and jmp to global labels in Intel syntax cause operand type mismatch errors. RCC emits .intel_syntax noprefix by default, but GAS ≥2.45 rejects direct branches to global symbols in this mode. Local labels (.L.xxx) work fine. Tests with user-defined function calls (bitops-1, fprintf-1, etc.) may fail to assemble under these versions. Root cause: rcc emits lea r11, [rip + sI] but GAS requires AT&T sI(%rip) for globals.