Teach rust core about Xtensa VaListImpl and add a custom lowering of vaarg for xtensa.

LLVM does not include an implementation of the va_arg instruction for
Xtensa. From what I understand, this is a conscious decision and
instead language frontends are encouraged to implement it themselves.
The rationale seems to be that loading values correctly requires
language and ABI-specific knowledge that LLVM lacks.

This is true of most architectures, and rustc already provides
implementation for a number of them. This commit extends the support to
include Xtensa.

See https://lists.llvm.org/pipermail/llvm-dev/2017-August/116337.html
for some discussion on the topic.

Unfortunately there does not seem to be a reference document for the
semantics of the va_list and va_arg on Xtensa. The most reliable source
is the GCC implementation, which this commit tries to follow. Clang also
provides its own compatible implementation.

This was tested for all the types that rustc allows in variadics.

Co-authored-by: Brian Tarricone <brian@tarricone.org>
Co-authored-by: Jonathan Bastien-Filiatrault <joe@x2a.org>
Co-authored-by: Paul Lietar <paul@lietar.net>

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