std.zig.system.x86: Synchronize CPUID/XGETBV checks with LLVM 19.
Also fix a bunch of cases where we didn't toggle features off if the relevant
leaf isn't available, and switch XCR0 checks to a packed struct.
Closes #23385.
7289a073e5Alex Rønne Petersen committed on 4/3/2025, 12:29:26 AMparentde62dc8