ci: run the full smoke test once, and split the GNUmakefile (#5616)

* GNUmakefile: split into topic files in make/

The GNUmakefile had 1295 lines and mixed build configuration, LLVM
bootstrapping, device generation, four test suites, the smoke tests,
release packaging, and lint tools. The smoke tests alone were 500 lines.

Move each part into its own file in make/ and include them from
GNUmakefile. config.mk must be included first because the other files
use its variables in immediate assignments and conditionals.

There is no change in behavior. The parsed make database is identical
for the default build and for ASSERT=1, STATIC=1, XTENSA=0, STM32=0,
WASM=0, and CROSS=aarch64-linux-gnu, except for MAKEFILE_LIST and the
.PHONY list, which now also includes targets that were phony but not
declared.

The Dockerfile copies GNUmakefile alone before it builds LLVM, to keep
that layer independent of the source tree. It must copy make/ too.

* make/smoketest.mk: split into groups and add smoketest-quick

The smoke test was one recipe of about 500 lines with 236 builds that
always ran in sequence. Split it at the group boundaries that were
already there, so that:

- `make -j smoketest` builds the groups in parallel. A full run goes
  from 325 to 91 seconds on a 32 core machine.
- CI can shard the groups across runners.

Each group writes to its own name in build/smoke/, because all builds
wrote to test.hex before and would overwrite each other in a parallel
build. The output extension selects the format, so it stays per line.

Add smoketest-quick, which builds one board for each processor
architecture. The full smoke test answers "can TinyGo build for every
board", which does not depend on the host OS, so it only needs to run
on one OS. The other jobs use smoketest-quick.

The comment above the esp32c3 group had 4 spaces of indentation instead
of a tab. That was harmless in the middle of a recipe, but it is now the
first line of a group, where it would stop the recipe from starting.

The set of build commands is unchanged for the default flags and for
XTENSA=0, STM32=0, and WASM=0. All 226 checksums are the same as before,
for a sequential build and for `make -j16`.

* ci: run the full smoke test once, on Linux only

The smoke test ran six times for each push: twice on Linux, twice on
macOS, once on Windows, and once in the compatibility test. Together
that was about 97 minutes of the CI time.

The smoke test checks that TinyGo can build a binary for each board.
That does not depend on the host OS. The only part that does is one
build behind a Windows check.

Add a smoketest-linux job that runs the full set, split across four
runners that use the tarball from the build-linux job. The groups are
balanced with the measured build time of each group. Remove the full
smoke test from test-linux-build, which the new job replaces, and use
smoketest-quick for the other four jobs.

Expected result: about 55 to 39 minutes for the slowest workflow, and
about 97 to 35 minutes of total smoke test time.

* make/smoketest.mk: build nintendoswitch in smoketest-quick

It is the only target that uses the aarch64 LLVM triple. With it,
smoketest-quick covers all 13 triples that the full smoke test uses.

BUILDING.md

@@ -30,7 +30,10 @@ on a different system like Mac.
 
 ## Using GNU Make
 
-The static build of TinyGo is driven by GNUmakefile, which provides a help target for quick reference:
+The static build of TinyGo is driven by GNUmakefile, which includes the topic
+files in the `make/` directory (`config.mk`, `llvm.mk`, `gen-device.mk`,
+`build.mk`, `test.mk`, `smoketest.mk`, `release.mk`, and `tools.mk`).
+It provides a help target for quick reference:
 
     % make help
     clean                           Remove build directory
@@ -64,7 +67,7 @@ while producing binaries that are about as fast.
     export CC=clang
     export CXX=clang++
 
-The Makefile includes a default configuration that is good for most users. It
+`make/config.mk` holds a default configuration that is good for most users. It
 builds a release version of LLVM (optimized, no asserts) and includes all
 targets supported by TinyGo: