Can't implement conformant memset/memcpy without compiler -ffreestanding support

[Zheng Luo (Vic)]

Current implementation of compilers assumes libc implementation, 
which leads to an infinite loop if we want to implement 
primitives like memset with our own code because the compiler 
will optimize consecutive set with "memset". This suggests that 
we cannot write a freestanding program without supports from 
compiler. With "-betterC" flag, ldc also comes into this issue, 
which also applies to C/C++[1] and rust [2][3][4].

gcc and clang provides an option "-ffreestanding" to bypass 
optimizations that need libc support. Although we can hack around 
this issue by making our implementation complicated enough/using 
assembly to bypass the optimizer, it would be better to provide a 
standard flag like "-ffreestanding" for all compilers to disable 
such optimizations, so that developers won't have to hack around 
different compiler implementations.

[1] https://godbolt.org/g/5gVWeN
[2] 
https://play.rust-lang.org/?gist=64f2acafa8cec112893633a5f2e12a9a&version=stable&mode=release&edition=2015
[3] https://github.com/rust-lang/rust/issues/10116
[4] https://github.com/thestinger/rust-core#freestanding