libc: add minimal support for AArch64

this contains still prototyped code, which if reached either
hangs (setjmp, longjmp), aborts (all that requires mmu::),
or implements differently (allocating with malloc instead of mmap)
This is enough libc AArch64 support for reaching the end of premain.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>