meson: Remove efi-cc option

Changing the efi compiler this way doesn't really work. The gnu-efi
header checks as well as supported compiler flag checks use the
regular cc that meson detects. Changing the compiler this way will
end up with bad compiler flags. For the very same reason, this does
not work with a cross-compiler without going through proper meson
cross-compilation steps either.

The proper way to build systemd-boot with a different compiler is to
use a different build folder and then just use the proper ninja build
target to only build the bootloader/stub.