meson: use the host architecture compiler/linker for src/boot/efi

meson: use the host architecture compiler/linker for src/boot/efi

cross building systemd to arm64 presently fails, because the build
system uses plain gcc and plain ld (build architecture compiler and
linker respectively) for building src/boot/efi. These values come from
the efi-cc and efi-ld options respectively. It rather should be using
host tools here.

Fixes: b710072da441 ("add support for building efi modules")