Enable gnu_f16_abi on x86_64

[argentite]

This fixes undefined reference to __gnu_h2f_ieee and __gnu_f2h_ieee on use of the std.fmt functions for example when building with: -target x86_64-freestanding-none -mcpu x86_64_v3-avx-avx2-f16c-fma-fxsr-mmx+soft_float-sse-sse2-sse3-sse4_1-sse4_2-ssse3-vzeroupper-x87-xsave -rdynamic Example code:

const std = @import("std");

var buffer: [1024 * 1024]u8 = undefined;
var alloc = std.heap.FixedBufferAllocator.init(buffer[0..buffer.len]);

pub export fn _start() void {
    const str = std.fmt.allocPrint(alloc.allocator(), "Hello", .{}) catch unreachable;
    defer alloc.allocator().free(str);
}

Thanks @matu3ba Related: #13333