stage2: implement zig build

As part of this:

 * add std.process.cleanExit. closes #6395
   - use it in several places
 * adjust the alignment of text in `zig build --help` menu
 * Cache: support the concept of "unhit" so that we properly keep track
   of the cache when we find out using the secondary hash that the cache
   "hit" was actually a miss. Use this to fix false negatives of caching
   of stage1 build artifacts.
 * fix not deleting the symlink hash for stage1 build artifacts causing
   false positives.
 * implement support for Package arguments in stage1 build artifacts
 * update and add missing usage text
 * add --override-lib-dir and --enable-cache CLI options
   - `--enable-cache` takes the place of `--cache on`
 * CLI supports -femit-bin=foo combined with --enable-cache to do an
   "update file" operation. --enable-cache without that argument
   will build the output into a cache directory and then print the path
   to stdout (matching master branch behavior).
 * errors surfacing from main() now print "error: Foo" instead of
   "error: error.Foo".

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/std/process.zig b/lib/std/process.zig
index 2813d8cbab..5303ada94b 100644
--- a/lib/std/process.zig
+++ b/lib/std/process.zig
@@ -19,6 +19,19 @@ pub const exit = os.exit;
 pub const changeCurDir = os.chdir;
 pub const changeCurDirC = os.chdirC;
 
+/// Indicate that we are now terminating with a successful exit code.
+/// In debug builds, this is a no-op, so that the calling code's
+/// cleanup mechanisms are tested and so that external tools that
+/// check for resource leaks can be accurate. In release builds, this
+/// calls exit(0), and does not return.
+pub fn cleanExit() void {
+    if (builtin.mode == .Debug) {
+        return;
+    } else {
+        exit(0);
+    }
+}
+
 /// The result is a slice of `out_buffer`, from index `0`.
 pub fn getCwd(out_buffer: []u8) ![]u8 {
     return os.getcwd(out_buffer);