[Support][Endian] Add support for specifying the alignment and native unaligned types.

* Add support for specifying the alignment to use.
* Add the concept of native endianness. Used for unaligned native types.

The native alignment and read/write simplification is based on a patch by Richard Smith.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171406 91177308-0d34-0410-b5e6-96231b3b80d8
Restrict __builtin_assume_aligned to gcc 4.7+

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171408 91177308-0d34-0410-b5e6-96231b3b80d8