bsd.endian.mk: Optimize the handling of big/little endian determination.

Add variables to contain lists of MACHINE_ARCH values to use
to determine little or big endian.

Only error out about not being able to determine endianess if
TARGET_ENDIANNESS is empty and not cross-compiling.

Reviewed by: sjg
Obtained from: Hewlett Packard Enterprise Development LP
Differential Revision: https://reviews.freebsd.org/D44629