some x86_64 self-hosted backend std lib tests are disabled

[andrewrk]

The CI has test coverage for x86 backend standard library tests, however some of them are disabled:

$ grep -RI 'stage2_x86_64.*Skip' lib/std/
lib/std/simd.zig:    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
lib/std/simd.zig:    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
lib/std/simd.zig:    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
lib/std/simd.zig:    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
lib/std/crypto/utils.zig:    if (@import("builtin").zig_backend == .stage2_x86_64) return error.SkipZigTest;
lib/std/math/big/int_test.zig:    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
lib/std/math/big/int_test.zig:    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
lib/std/hash/xxhash.zig:    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
lib/std/hash/xxhash.zig:    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
lib/std/hash/xxhash.zig:    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;

Once the above list becomes small enough, this issue can be split into several follow-up issues.

This is a prerequisite to making the x86_64 self-hosted backend the default backend for debug builds.

Related:

• x86_64 self-hosted backend universal-libc tests are failing #15328