Add regression test for #2666.

I didn't see how to run it easily from main_test.go, though I didn't try too hard.
And it doesn't really have a good place to go in Makefile.
So I added a new target tinygo-baremetal, and invoke it from CI at the end of assert-test-linux.
It only adds 7 seconds to the run, should be ok.

.github/workflows/linux.yml

@@ -239,3 +239,4 @@ jobs:
           rm xtensa-esp32-elf-gcc8_2_0-esp-2020r2-linux-amd64.tar.gz
       - run: make smoketest
       - run: make wasmtest
+      - run: make tinygo-baremetal

Makefile

@@ -331,6 +331,10 @@ test-corpus-fast:
 test-corpus-wasi: wasi-libc
 	CGO_CPPFLAGS="$(CGO_CPPFLAGS)" CGO_CXXFLAGS="$(CGO_CXXFLAGS)" CGO_LDFLAGS="$(CGO_LDFLAGS)" $(GO) test $(GOTESTFLAGS) -timeout=1h -buildmode exe -tags byollvm -run TestCorpus . -corpus=testdata/corpus.yaml -target=wasi
 
+tinygo-baremetal:
+	# Regression tests that run on a baremetal target and don't fit in either main_test.go or smoketest.
+	# regression test for #2666: e.g. encoding/hex must pass on baremetal
+	$(TINYGO) test -target cortex-m-qemu encoding/hex
 
 .PHONY: smoketest
 smoketest: