tool: replace go.cmd with a 19KB Rust go.exe wrapper

go.cmd used cmd.exe to invoke PowerShell, which mangled arguments:
cmd.exe treats ^ as an escape character (so -run "^$" became -run "$",
running all tests instead of none) and = signs also caused issues in
the PowerShell→cmd.exe argument passing layer.

Replace it with a tiny no_std Rust binary (19KB, 32-bit x86 for
universal Windows compat: x86/x64/ARM64) that directly invokes the
Tailscale Go toolchain via CreateProcessW. The raw command line from
GetCommandLineW is passed through to CreateProcessW with only argv[0]
replaced, so arguments are never parsed or re-escaped.

The binary also handles first-run toolchain download natively using
curl.exe and tar.exe (both ship with Windows 10+), so PowerShell is
no longer required for normal operation. The PowerShell fallback is
only used for the rare TS_USE_GOCROSS=1 path.

PowerShell prefers go.exe over go.cmd when resolving ./tool/go, so
this is a drop-in replacement.

With go.exe in place, the CI can use the natural -bench=. -benchtime=1x
-run="^$" flags directly.

Also removes tool/go-win.ps1 which is now unused.

Updates #19255

Change-Id: I80da23285b74796e7694b89cff29a9fa0eaa6281
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tool/goexe/Makefile

@@ -0,0 +1,28 @@
+# Builds tool/go.exe, a thin wrapper that execs the Tailscale Go
+# toolchain without going through cmd.exe (which mangles ^ and other
+# special characters in arguments).
+# See https://github.com/tailscale/tailscale/issues/19255
+#
+# Built as no_std Rust with raw Win32 API calls for minimal size (~17KB).
+# The resulting go.exe is checked into the repo at tool/go.exe.
+#
+# Built as 32-bit x86 so one binary runs on x86, x64 (via WoW64),
+# and ARM64 (via Windows x86 emulation).
+#
+# Requirements:
+#   rustup target add i686-pc-windows-gnu
+#   apt install gcc-mingw-w64-i686  (or equivalent)
+
+RUST_TARGET = i686-pc-windows-gnu
+
+.PHONY: all clean
+
+all: go.exe
+
+go.exe: src/main.rs Cargo.toml
+	cargo build --release --target $(RUST_TARGET)
+	cp target/$(RUST_TARGET)/release/go.exe $@
+
+clean:
+	rm -f go.exe
+	rm -rf target