delete all stage1 c++ code not directly related to compiling stage2

Deleted 16,000+ lines of c++ code, including:
 * an implementation of blake hashing
 * the cache hash system
 * compiler.cpp
 * all the linking code, and everything having to do with building
   glibc, musl, and mingw-w64
 * much of the stage1 compiler internals got slimmed down since it
   now assumes it is always outputting an object file.

More stuff:
 * stage1 is now built with a different strategy: we have a tiny
   zig0.cpp which is a slimmed down version of what stage1 main.cpp used
   to be. Its only purpose is to build stage2 zig code into an object
   file, which is then linked by the host build system (cmake) into
   stage1. zig0.cpp uses the same C API that stage2 now has access to,
   so that stage2 zig code can call into stage1 c++ code.
   - stage1.h is
   - stage2.h is
   - stage1.zig is the main entry point for the Zig/C++
     hybrid compiler. It has the functions exported from Zig, called
     in C++, and bindings for the functions exported from C++, called
     from Zig.
 * removed the memory profiling instrumentation from stage1.
   Abandon ship!
 * Re-added the sections to the README about how to build stage2 and
   stage3.
 * stage2 now knows as a comptime boolean whether it is being compiled
   as part of stage1 or as stage2.
   - TODO use this flag to call into stage1 for compiling zig code.
 * introduce -fdll-export-fns and -fno-dll-export-fns and clarify
   its relationship to link_mode (static/dynamic)
 * implement depending on LLVM to detect native target cpu features when
   LLVM extensions are enabled and zig lacks CPU feature detection for
   that target architecture.
 * C importing is broken, will need some stage2 support to function
   again.

README.md

@@ -22,6 +22,8 @@ Note that you can
 
 ### Stage 1: Build Zig from C++ Source Code
 
+This step must be repeated when you make changes to any of the C++ source code.
+
 #### Dependencies
 
 ##### POSIX
@@ -77,6 +79,41 @@ Hopefully this will be fixed upstream with LLVM 10.0.1.
 
 See https://github.com/ziglang/zig/wiki/Building-Zig-on-Windows
 
+### Stage 2: Build Self-Hosted Zig from Zig Source Code
+
+Now we use the stage1 binary:
+
+```
+zig build --prefix $(pwd)/stage2 -Denable-llvm
+```
+
+This produces `stage2/bin/zig` which can be used for testing and development.
+Once it is feature complete, it will be used to build stage 3 - the final compiler
+binary.
+
+### Stage 3: Rebuild Self-Hosted Zig Using the Self-Hosted Compiler
+
+*Note: Stage 2 compiler is not yet able to build Stage 3. Building Stage 3 is
+not yet supported.*
+
+Once the self-hosted compiler can build itself, this will be the actual
+compiler binary that we will install to the system. Until then, users should
+use stage 1.
+
+#### Debug / Development Build
+
+```
+stage2/bin/zig build
+```
+
+This produces `zig-cache/bin/zig`.
+
+#### Release / Install Build
+
+```
+stage2/bin/zig build install -Drelease
+```
+
 ## License
 
 The ultimate goal of the Zig project is to serve users. As a first-order