decl: Allow union specifiers to contain structs/unions with flexible array members

Thanks to Thorsten Glaser for reporting this issue.

Fixes #91.

decl: Fix divide-by-zero with VLA of constant length and VLA element

These are VLAs as well, so we should just leave size set to 0. It
will be calculated in calcvla() like other VLAs.

Thanks to Davide Di Paolo for reporting this issue.

init: Fix check for VLA types

A VLA type can be incomplete (for example `int x[][n]`), which was
not previously detected here, causing an assertion failure later
on.

runtests: Don't include invalid tests in results

Add defineobj() function and use when we define an object

This gives us a place to update alignment based on the final type,
which is relevant for external definitions and tentative definitions.
This fixes a bug where an object is declared with an incomplete
struct type, the struct type is completed, and then the object is
defined. This would previously result in an object declared with 0
alignment.

Also, check for incomplete types when we are defining objects.
Although the wording isn't clear in the C standard, and it doesn't
seem to disallow a thread_local definition of an object with
incomplete type, this doesn't make sense.

Add missing section comment

eval: Reuse decl from compound expression parsing

In 593ca07956, we started creating a decl for compound literals
during expression parsing, which was used by funclval(). However,
eval() also created a declaration, which should have been changed
at the same time.

Store enum constant value in struct decl

Implement variable length arrays

Variably modified types are required for C23.

Since QBE doesn't currently support saving and restoring the stack
pointer, a current limitation is that we can't reclaim stack space
from VLAs that go out of scope. This is potentially problematic for
VLAs appearing in a loop, but this case is uncommon enough that it
is silently ignored for now.

Implements: https://todo.sr.ht/~mcf/cproc/1
References: https://todo.sr.ht/~mcf/cproc/88
Co-authored-by: Michael Forney <mforney@mforney.org>

qbe: Add extra size for strict alignment with QBE instruction

With upcoming VLA support, the size might not be constant.

qbe: Temporarily set func->end = func->start in funcalloc

This way way can just use funcinst.

Use hlt to implement noreturn

expr: Collapse *& when parsing

qbe: Use expression type when loading

If we are loading the first element of an array, or first member
of a structure, the expression type may be different from the object
type.

decl: Move isnoreturn to struct decl

It is a property of the declaration, not the type.

configure: Set DEFAULT_DYNAMIC_LINKER for riscv64-linux-gnu

README: Add riscv64 to supported targets

CI: Add debian-riscv64 builder

CI: Simplify alpine-aarch64.yml and debian-aarch64.yml

CI: Remove obsolete workaround for qemu-binfmt bug