Support `vgetrandom` on Linux 6.11+

[The-King-of-Toasters]

Linux 6.11 added getrandom to the vdso. The supported architectures are:

• x86_64
• aarch64
• loongarch64
• s390x
• powerpc64(le)

The API is:

ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags,
                   void *opaque_state, size_t opaque_len);
// If buffer = len = flags = 0 && opaque_len = ~0, opaque_state is set to the struct:
struct vgetrandom_opaque_params {
    __u32 size_of_opaque_state;
    __u32 mmap_prot;
    __u32 mmap_flags;
    __u32 reserved[13];
};

Note that it isn't as simple as calling vgetrandom, callers need to pre-allocate memory to pass to the syscall.

Go recently implemented support in golang/go/issues/69577 (see https://go-review.googlesource.com/c/go/+/614835 for the patchset). Note that this impl allocates a pool of states per the number of CPUs, and uses a lock to fetch an unused state.

[The-King-of-Toasters]

Tagging @jedisct1 since this effects the std.crypto namespace.