Github-CI: add linux 32-bit target (DO NOT MERGE)

[chqrlie]

• enable linux 32-bit target
• fix 32-bit issues
• add 32-bit struct sizes in static_assert validations
• create bootstrap/bootstrap-linux-i686.patch

[bvdberg]

I think the static_assert(sizeof(void*) == 4 ? 132 : 144+64, sizeof(AST)); is an anti-pattern.
It would be better to extend the static_assert syntax to support this natively.

Options I can think of:

static_assert(8, sizeof(void*)); // generic case
static_assert[32](4, sizeof(void*)); // 32 bit platforms only
static_assert[64](8, sizeof(void*)); // 64 bit platforms only

// have different static assert functions (also the generic one)
static_assert32(..);
static_assert64(..);

// if needed (not now) we could extend the first syntax to include arch/system limits. The list of these is already in common/target_info.c2.
static_assertarch=arm64
static_assertarch=arm64,system=darwin

I think the arch/system options might be needed in the future, so perhaps the [] syntax is the better choice..