> Does this mean the Linux x86_64 kernel could only be built with gcc before this LLVM release?

It was already possible to compile the kernel with clang/llvm. But a few releases ago, the Linux kernel dropped support for being built with older compilers which did not have support for the "asm goto" gcc extension.

The Linux kernel has always targeted gcc exclusively, and makes heavy use of gcc extensions since its very first release. The only reason clang can compile the Linux kernel is that the clang developers implemented support for the many gcc extensions the Linux kernel uses.