bp2build: Add special arch-variant targets.

Soong supports additional arch-variant targets based on arch/os aside
from strictly arch/os names. Handle the unconverted cases based solely
on arch/os.

Test: build/bazel/ci/bp2build.sh
Change-Id: I8dc987b1aa4a4ea646dd5b03a94b84297ba6113a

bp2build/cc_library_conversion_test.go

@@ -133,8 +133,8 @@ cc_library {
         "//conditions:default": [],
     }) + select({
         "//build/bazel/platforms/os:android": [
-            "android.cpp",
             "bionic.cpp",
+            "android.cpp",
         ],
         "//build/bazel/platforms/os:darwin": ["darwin.cpp"],
         "//build/bazel/platforms/os:linux": ["linux.cpp"],
@@ -1668,22 +1668,22 @@ cc_library {
     name = "foo-lib",
     srcs = ["base.cpp"] + select({
         "//build/bazel/platforms/os:android": [
-            "android.cpp",
             "linux.cpp",
             "bionic.cpp",
+            "android.cpp",
         ],
         "//build/bazel/platforms/os:darwin": ["darwin.cpp"],
         "//build/bazel/platforms/os:linux": [
-            "linux_glibc.cpp",
             "linux.cpp",
+            "linux_glibc.cpp",
         ],
         "//build/bazel/platforms/os:linux_bionic": [
             "linux.cpp",
             "bionic.cpp",
         ],
         "//build/bazel/platforms/os:linux_musl": [
-            "linux_musl.cpp",
             "linux.cpp",
+            "linux_musl.cpp",
         ],
         "//build/bazel/platforms/os:windows": ["windows.cpp"],
         "//conditions:default": [],