loader: anonymous mmap passes fd `0` and fails with `EINVAL` on BSD

Describe the bug

loader/mmap_unix.go currently invokes SYS_MMAP with MAP_ANON | MAP_PRIVATE but passes 0 as the fd argument:

syscall.RawSyscall6(
    syscall.SYS_MMAP,
    0,
    uintptr(nb),
    _RW,
    _AP,
    0, // fd
    0, // offset
)

This causes JIT memory allocation to fail with EINVAL on BSD distros.

The earlier fix for #564, merged as #567, changed the build constraints so that this implementation is selected on additional non-Windows systems. It resolved the compilation failure, but exposed a runtime incompatibility in the mmap arguments.

Platform compatibility

Using -1 is the portable choice for platforms on which this POSIX/BSD-style anonymous mmap implementation is used:

• FreeBSD requires fd == -1 and offset == 0 for MAP_ANON. FreeBSD mmap(2)
• OpenBSD requires fd == -1. OpenBSD mmap(2)
• DragonFly BSD requiresfd == -1. DragonFly mmap(2)
• Linux ignores fd for MAP_ANONYMOUS, but its documentation recommends -1 for portable applications and says the offset should be zero. man7.org Linux mmap(2)
• Darwin permits -1 when no Mach VM flags are being supplied through the descriptor argument. Apple Documentation Archive mmap(2)

Consequently, changing the descriptor from 0 to -1 fixes BSD issue without changing the anonymous-mapping behavior on Linux or macOS.

Expected behavior

For an anonymous mapping, the call should use:

mmap(NULL, pagesize, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0);

In a raw Go syscall, -1 can be represented as ^uintptr(0)

Sonic version: latest

Additional context

Related work:

• #564 reported that the mmap implementation was not selected for BSD targets.
• #567 broadened the build constraint, but did not change the mmap descriptor.
• #902 contains a proposed descriptor fix and appears to address this runtime issue.