Allow installing Alpine's mesa for x86* arches (!1768)

When a pmaport can not be built for the desired architecture, fall back
to the binary package (from postmarketOS or Alpine) if it exists. This
allows us to provide an updated version of mesa for arm arches, but
using Alpine's mesa package on x86* arches.

test/test_build_package.py

@@ -102,7 +102,7 @@ def test_get_apkbuild(args):
     assert "Could not find" in str(e.value)
 
 
-def test_check_arch_abort(monkeypatch, args):
+def test_check_build_for_arch(monkeypatch, args):
     # Fake APKBUILD data
     apkbuild = {"pkgname": "testpkgname"}
 
@@ -110,21 +110,28 @@ def test_check_arch_abort(monkeypatch, args):
         return apkbuild
     monkeypatch.setattr(pmb.helpers.pmaports, "get", fake_helpers_pmaports_get)
 
-    # Arch is right
-    func = pmb.build._package.check_arch_abort
+    # pmaport with arch exists
+    func = pmb.build._package.check_build_for_arch
     apkbuild["arch"] = ["armhf"]
-    func(args, "testpkgname", "armhf")
+    assert func(args, "testpkgname", "armhf") is True
     apkbuild["arch"] = ["noarch"]
-    func(args, "testpkgname", "armhf")
+    assert func(args, "testpkgname", "armhf") is True
     apkbuild["arch"] = ["all"]
-    func(args, "testpkgname", "armhf")
+    assert func(args, "testpkgname", "armhf") is True
 
-    # Arch is wrong
+    # No binary package exists and can't build it
     apkbuild["arch"] = ["x86_64"]
     with pytest.raises(RuntimeError) as e:
         func(args, "testpkgname", "armhf")
     assert "Can't build" in str(e.value)
 
+    # pmaport can't be built for x86_64, but binary package exists in Alpine
+    apkbuild = {"pkgname": "mesa",
+                "arch": "armhf",
+                "pkgver": "9999",
+                "pkgrel": "0"}
+    assert func(args, "mesa", "x86_64") is False
+
 
 def test_get_depends(monkeypatch):
     func = pmb.build._package.get_depends