nativeGen PPC: fix > 16 bit offsets in stack handling

Implement access to spill slots at offsets larger than 16 bits.
Also allocation and deallocation of spill slots was restricted to
16 bit offsets. Now 32 bit offsets are supported on all PowerPC
platforms.

The implementation of 32 bit offsets requires more than one instruction
but the native code generator wants one instruction. So we implement
pseudo-instructions that are pretty printed into multiple assembly
instructions.

With pseudo-instructions for spill slot allocation and deallocation
we can also implement handling of the back chain pointer according
to the ELF ABIs.

Test Plan: validate (especially on powerpc (32 bit))

Reviewers: bgamari, austin, erikd

Reviewed By: erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1296

GHC Trac Issues: #7830