toolchain: fix __BYTE_ORDER__ not defined for XCC

Commit 571741a0 introduced a block to determine __BYTE_ORDER__
within toolchain/gcc.h. However, toolchain/xcc.h defined
__BYTE_ORDER__ there but the early inclusion of gcc.h causes
errors since __BYTE_ORDER__ cannot be determined within gcc.h.
So wrap around the gcc.h include with a fake __BYTE_ORDER__ to
bypass the check in gcc.h.

Also set the __BYTE_ORDER__ to the correct __ORDER_*_ENDIAN__
macro.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>