PUBLIC is a MASM directive and causes a build error in a certain version of MSVC
```
Microsoft (R) ARM Macro Assembler Version 14.44.35217.0 for 64 bits
Copyright (C) Microsoft Corporation. All rights reserved.
D:\r\_work\swift-build\swift-build\SourceCache\swift\stdlib\public\RuntimeModule\get-cpu-context-aarch64.asm(21) : error A2003: improper line syntax: PUBLIC
```
EXPORT is an ARMASM equivalent, which is already there.
Tweaked the comment in `Runtime/Config.h`.
Fixed a couple of incorrect ARM64 instruction mnemonics. This still needs
testing on ARM64 Windows.
Fixed an out-of-date comment in `swift-backtrace`.
Use a macro in `Backtrace.cpp` to guarantee we don't overrun the buffer,
and in the process simplify the code slightly.
rdar://101623384
On-crash backtracing is basically there for 64-bit Windows. It
won't work on 32-bit because of a Swift compiler issue, and there
is a little more work to do yet, but it is now working!
rdar://101623384
This doesn't have a working symbolicator yet, but it does build and
it can obtain a basic backtrace.
It also doesn't include a working `swift-backtrace` program yet.
rdar://101623384
Hiroshi Yamauchi
Alastair Houghton