fix[bsd]: MAP_ANON should be set to -1 , ignored on Linux, required on BSD + go fmt

[gboddin]

• fix ( mmap_unix.go missing BSD in Go build tag #564 , closed but still present on freebsd )

Description

This PR fixes a panic occurring on FreeBSD (and potentially macOS/other BSDs) where mmap fails with EINVAL (invalid argument).

The Issue

Currently, mmap_unix.go passes 0 as the file descriptor (fd) when calling syscall.SYS_MMAP with MAP_ANON.

• On Linux: The kernel ignores the fd argument for anonymous mappings, so passing 0 works fine.
• On FreeBSD: The kernel strictly adheres to POSIX behavior for MAP_ANON, which requires the fd to be -1. Passing 0 results in an immediate crash.

Panic Output (FreeBSD):

panic: invalid argument
goroutine 1 [running]:
[github.com/bytedance/sonic/loader.mmap](https://github.com/bytedance/sonic/loader.mmap)(...)
    .../loader/mmap_unix.go:35 +0x7b

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