Add x86-64 checking for Darwin

Detect byte endianness when compiled in x86-64 mode in Mac OS X 10.6
Snow Leopard environments or above. Allows compilation of host binaries
in these environments without any changes from its default stock
configuration.

This change doesn't require that the binaries be built in 64-bit, but it
does allow this. One could still use "gcc_select 4.0" with the 10.4 SDK
installed on Snow Leopard to compile in 32-bit mode.

Change-Id: I155a531a9bff450dd6c077b5275652731c59a908