PR29961, plugin-api.h: "Could not detect architecture endianess"

Found when attempting to build binutils on sparc sunos-5.8 where
sys/byteorder.h defines _BIG_ENDIAN but not any of the BYTE_ORDER
variants.  This patch adds the extra tests to cope with the old
machine, and tidies the header a little.

	PR 29961
	plugin-api.h: When handling non-gcc or gcc < 4.6.0 include
	necessary header files before testing macros.  Make more use
	of #elif.  Test _LITTLE_ENDIAN and _BIG_ENDIAN in final tests.