run astyle on all C files in the repository

Leaves parse tests and generated files unchanged, and provides a style
filter for these.

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

licenses: convert license tags to SPDX

Handling of AsmStmt in c-parser, more tests.

The C-parser contains a full parser for __asm__ syntax but
up until now hasn't done anything with it. Instead we export
some semantics. It's unspecified exactly what these semantics
are but they are parametrised with the __asm__ semantics that
went in to them, so the translation validation has something
to reason about.

Tweak modifies proofs as a result, and add some more test files.