[PowerPC] Support the nest parameter attribute

[PowerPC] Support the nest parameter attribute

This adds support for the 'nest' attribute, which allows the static chain
register to be set for functions calls under non-Darwin PPC/PPC64 targets. r11
is the chain register (which the PPC64 ELF ABI calls the "environment
pointer"). For indirect calls under PPC64 ELFv1, this would normally be loaded
from the function descriptor, but providing an explicit 'nest' parameter will
override that process and use the value provided.

This allows __builtin_call_with_static_chain to work as expected on PowerPC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241984 91177308-0d34-0410-b5e6-96231b3b80d8