arm64: initial find_string support

not compile tested yet
arm64: find_string works

part of the problem was the open() syscall is apparently not
available on arm64 and we had to use openat() instead.
arm64: printing RAM size works

had to fix the offset into the sysinfo structure to
take into account 64-bit
arm64: fix fakeproc cpuinfo file

the filename was one character too small
arm64: start optimizing

free up a few bytes by removing start_bss and data_bss from dedicated
registers.  that hack didn't buy us anything anymore, unlike on arm32
arm64: make first line code a bit smaller

use a half-word store rather than two byte stores
arm64: optimize find_string

Optimize the branches, also use cbz instructions
arm64: optimize strcat

use the cbnz instruction
arm64: optimize write_stdout

Use cbnz
arm64: move strcat to be a tail call
arm64: update documentation