Fix EFI loader object tree creation on 9.x build hosts

Previously ${COMPILER_TYPE} was checked in sys/boot/amd64, and the efi
subdirectory was skipped altogether for gcc (since GCC does not support
a required attribute). However, during the early buildworld stages
${COMPILER_TYPE} is the existing system compiler (i.e., gcc on 9.x build
hosts), not the compiler that will eventually be used. This caused
"make obj" to skip the efi subdirectory. In later build stages
${COMPILER_TYPE} is "clang", and then the efi loader would attempt to
build in the source directory.

Sponsored by: The FreeBSD Foundation