Add TargetLoweringInfo hook for explicitly setting the ABI calling convention endianess

Summary:
The endianess used in the calling convention does not always match the
endianess of the target on all architectures, namely AVR.

When an argument is too large to be legalised by the architecture and is
split for the ABI, a new hook TargetLoweringInfo::shouldSplitFunctionArgumentsAsLittleEndian
is queried to find the endianess that function arguments must be laid
out in.

This approach was recommended by Eli Friedman.

Originally reported in https://github.com/avr-rust/rust/issues/129.

Patch by Carl Peto.

Reviewers: bogner, t.p.northover, RKSimon, niravd, efriedma

Reviewed By: efriedma

Subscribers: JDevlieghere, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62003

llvm-svn: 361222