build: add -Donly-c option

[andrewrk]

This option can be used to produce a C backend build of the self-hosted compiler, which only has the C backend enabled. Once the C backend is capable of self-hosting, this will be a way for us to replace our stage1 codebase with a C backend build of self-hosted, which we can then use for bootstrapping. See #5246 for more details.

Using this option right now results in a crash because the C backend is not yet passing all the behavior tests.

[kubkon]

Very exciting!

[jacobly0]

Just FYI this can also trigger some compile errors when enabled:

src/Compilation.zig:2454:9: note: 'error.FlushFailure' not a member of destination error set
src/Compilation.zig:2455:9: note: 'error.LLDReportedFailure' not a member of destination error set

[andrewrk]

Thanks for pointing this out - fixed in #13294.

After this, the unique errors that remain to build Zig with -Donly-c in your branch are these:

/home/andy/dev/zig/lib/std/multi_array_list.zig:438:9: error: TODO: C backend: implement value of type Vector
        fn capacityInBytes(capacity: usize) usize {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/andy/dev/zig/src/link.zig:624:9: error: TODO: C backend: implement airFieldParentPtr
    pub fn destroy(base: *File) void {
    ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/andy/dev/zig/lib/std/child_process.zig:496:5: error: TODO: C backend: implement cmp_lt_errors_len
    fn cleanupAfterWait(self: *ChildProcess, status: u32) !Term {
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/andy/dev/zig/src/value.zig:1615:5: error: TODO: C backend: implement lowering large float values
    fn bigIntToFloat(limbs: []const std.math.big.Limb, positive: bool) f128 {
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/andy/dev/zig/lib/std/fmt/parse_float/parse_float.zig:14:5: error: TODO float types > 64 bits are not support in renderValue() as of now
pub fn parseFloat(comptime T: type, s: []const u8) ParseFloatError!T {
~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

😮