llvm-efi is a Bazel cc toolchain that uses the sandbox sysroot LLVM/clang with flags targeting freestanding EFI. EFI headers are not shipped as part of the toolchain, but are available as a cc_library from @gnuefi//:gnuefi.
To use this toolchain explicitly while building a cc_binary, do:
bazel build --platforms=//build/platforms:efi_amd64 //foo/bar
During an actual build however, the right toolchain should be selected using transitions or other configuration mechanisms.
This is a special symbol emitted by MSVC-compatible compilers. In an EFI environment it can be ignored, but it needs to be defined. See fltused.c for more information on the symbol. Since we cannot build an object file with Bazel and building things for toolchains isn't a thing anyways, this file is prebuilt. If this ever needs to be rebuilt (which will probably never happen since there is only one static symbol in there) this can be done with the following clang invocation:
clang -target x86_64-unknown-windows -fno-ms-compatibility -fno-ms-extensions -ffreestanding -o fltused.o .o -c fltused.c