compiler.h: use u-boot endian macros on OpenBSD

When building u-boot on sparc64 and powerpc hosts it became clear that
u-boot expects endian conversion defines to be macros:

lib/crc32.c:87: error: braced-group within expression allowed only inside a function

For OpenBSD switch from using system definitions equivalent to the u-boot ones
and define glibc __BYTE_ORDER __BIG_ENDIAN __LITTLE_ENDIAN names, as at least
some parts of the non-cross build assumes those names are present (ie crc32.c).

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>