Fix warnings
auto merge of #5653 : brson/rust/warnings, r=brson
rt/arch/arm: fix syntax used for noexec stack
auto merge of #5655 : thestinger/rust/arm, r=brson

Turns out @ isn't valid for the ARM assembler.
core: Update libc docs to clarify usage
doc: Update tutorial description of core
auto merge of #5660 : brson/rust/doc, r=catamorphism