Do not emit AVX instructions for UEFI target

[justinian]

While working on porting my bootloader to Zig, I hit issues with #UD (undefined opcode) exceptions in my program. The following minimal Zig code will cause the error when not compiled in one of the release modes.

const std = @import("std");
const uefi = std.os.uefi;

pub fn main() uefi.Status {
    var buf: [64]u16 = undefined;
    while (true) {}
}

The issue is with the following assembly, which looks to be doing a memset-like routine on buf using AVX isntructions:

// main()
 470:   55                      push   rbp
 471:   48 81 ec b0 00 00 00    sub    rsp,0xb0
 478:   48 8d ac 24 80 00 00    lea    rbp,[rsp+0x80]
 47f:   00
 480:   48 8d 45 a8             lea    rax,[rbp-0x58]
 484:   48 89 c1                mov    rcx,rax
 487:   ba aa 00 00 00          mov    edx,0xaa
 48c:   41 b8 80 00 00 00       mov    r8d,0x80
 492:   e8 b9 00 00 00          call   0x550
 497:   eb fe                   jmp    0x497
 499:   0f 1f 80 00 00 00 00    nop    DWORD PTR [rax+0x0]

// memset
 550:   48 89 c8                mov    rax,rcx
 553:   4d 85 c0                test   r8,r8
 556:   74 64                   je     0x5bc
 558:   49 81 f8 80 00 00 00    cmp    r8,0x80
 55f:   73 05                   jae    0x566
 561:   45 31 c9                xor    r9d,r9d
 564:   eb 4a                   jmp    0x5b0
 566:   c5 f9 6e c2             vmovd  xmm0,edx
 56a:   4d 89 c1                mov    r9,r8
 56d:   31 c9                   xor    ecx,ecx
 56f:   49 83 e1 80             and    r9,0xffffffffffffff80
 573:   c4 e2 7d 78 c0          vpbroadcastb ymm0,xmm0    // ===== #UD EXCEPTION HERE =====
 578:   0f 1f 84 00 00 00 00    nop    DWORD PTR [rax+rax*1+0x0]
 57f:   00
 580:   c5 fe 7f 04 08          vmovdqu YMMWORD PTR [rax+rcx*1],ymm0
 585:   c5 fe 7f 44 08 20       vmovdqu YMMWORD PTR [rax+rcx*1+0x20],ymm0
 58b:   c5 fe 7f 44 08 40       vmovdqu YMMWORD PTR [rax+rcx*1+0x40],ymm0
 591:   c5 fe 7f 44 08 60       vmovdqu YMMWORD PTR [rax+rcx*1+0x60],ymm0
 597:   48 83 e9 80             sub    rcx,0xffffffffffffff80
 59b:   49 39 c9                cmp    r9,rcx
 59e:   0f 85 dc ff ff ff       jne    0x580
 5a4:   4d 39 c1                cmp    r9,r8
 5a7:   0f 84 0f 00 00 00       je     0x5bc
 5ad:   0f 1f 00                nop    DWORD PTR [rax]
 5b0:   42 88 14 08             mov    BYTE PTR [rax+r9*1],dl
 5b4:   49 ff c1                inc    r9
 5b7:   4d 39 c8                cmp    r8,r9
 5ba:   75 f4                   jne    0x5b0
 5bc:   c5 f8 77                vzeroupper
 5bf:   c3                      ret

Since enabling AVX isn't a part of the state for x64 UEFI apps defined by the spec, this target should probably default to not using AVX.