Boehm-GC: fix global const handling (#9477)

* define global `__v_inside_init`

* unset `__v_inside_init` after `_vinit()`

* define `C.GC_MALLOC_UNCOLLECTABLE()`

* allocate uncollectable memory during `_vinit()`

* ci: run test cases with V copiler that uses GC-Boehm itself

* wrap `__v_inside_init` access into `#ifdef _VGCBOEHM`

.github/workflows/ci.yml

@@ -129,7 +129,9 @@ jobs:
           thirdparty/tcc/tcc.exe -version
           ./v -cg -o v cmd/v # Make sure vtcc can build itself twice
       - name: v self compilation with -gc boehm
-        run: ./v -gc boehm -o v2 cmd/v && ./v2 -gc boehm -o v3 cmd/v && ./v3 -gc boehm -o v4 cmd/v
+        run: |
+          ./v -gc boehm -o v2 cmd/v && ./v2 -gc boehm -o v3 cmd/v && ./v3 -gc boehm -o v4 cmd/v
+          mv v4 v
       - name: v doctor
         run: |
           ./v doctor
@@ -137,8 +139,7 @@ jobs:
         run: |
           ./v -gc boehm cmd/tools/test_if_v_test_system_works.v
           ./cmd/tools/test_if_v_test_system_works
-      - name: Self tests with `-gc boehm`
-        ## The test cases are run with non-gc `v` for now
+      - name: Self tests with `-gc boehm` with V compiler using Boehm-GC itself
         run: ./v -gc boehm -silent test-self
       - name: Test leak detector
         run: |