powerpc: use default endianness for converting guest/init

For converting the guest/init binary into an object file, we call
the linker binary, setting the endianness to big endian explicitly
when compiling kvmtool for powerpc.
This breaks if the compiler is actually targetting little endian
(which is true for the Debian port, for instance).
Remove the explicit big endianness switch from the linker call to
allow linking on little endian PowerPC builds again.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>