enable the x86 backend by default for debug builds on x86_64-linux

[andrewrk]

Blockers:

• some x86_64 self-hosted backend std lib tests are disabled #15329
• x86 backend is not passing all behavior tests #17645
• x86 backend: support all the frontend features #19372
• x86_64: Bug multiplying @as(u17, @intCast(u8)) * comptime_int #19607
• Missing x86 mnemonics and registers (self-hosted backend encoder) #19773
• x86_64 backend miscomp #20113
• x86_64 backend genSetReg called with a value larger than dst_reg #20122
• x86 backend miscompilation w/ packed structs and bitwise operation #20581
• x86 backend compile bug: error(x86_64_encoder): no encoding found for: none vsubss xmm0 xmm0 r32 none #20955
• link.Dwarf loads AST when emitting debug info #21227
• self-hosted x86 backend has formatted panics disabled #21529
• implement the rest of the language features in the self-hosted x86 backend #21530
• panic: switch on corrupt value running code generated by self-hosted x86_64 backend #21834
• selfhosted x86_64: no encoding found for: none imul r8 r8 none none #21950