Seperate asflags and cflags

This fixes a bug which was a misunderstanding of soong properties:
Soong's cflags pertain only to C and C++ language, whereas bazel's copts
pertain to all three languages. This change ensures that asflags are
added as specifically asflags, and the 'copts' for the static library
macro pertains only to C and C++ languages.

This requires a somewhat hacky workaround for asflags, however: Since
assembly sources also need includepath-related flags, this duplicates
these flags between copts and asflags. To reduce verbosity of
bp2build-generated targets, this also ensures that asflags are omitted
in cases where there are no assembly sources.

Test: Mixed build droid CI
Change-Id: Ic0babed1f90d6dc82e5788638681ce5b995043f8
diff --git a/bp2build/cc_object_conversion_test.go b/bp2build/cc_object_conversion_test.go
index 8ede226..df4924b 100644
--- a/bp2build/cc_object_conversion_test.go
+++ b/bp2build/cc_object_conversion_test.go
@@ -211,6 +211,7 @@
             asflags: ["-DPLATFORM_SDK_VERSION=%d"],
         },
     },
+    srcs: ["src.S"],
 }
 `,
 		expectedBazelTargets: []string{`cc_object(
@@ -220,6 +221,7 @@
         "//conditions:default": [],
     }),
     copts = ["-fno-addrsig"],
+    srcs_as = ["src.S"],
 )`,
 		},
 	})
@@ -240,7 +242,7 @@
             cflags: ["-fPIC"], // string list
         },
         arm: {
-            srcs: ["arch/arm/file.S"], // label list
+            srcs: ["arch/arm/file.cpp"], // label list
         },
     },
 }
@@ -257,7 +259,7 @@
         "//conditions:default": [],
     }),
     srcs = ["a.cpp"] + select({
-        "//build/bazel/platforms/arch:arm": ["arch/arm/file.S"],
+        "//build/bazel/platforms/arch:arm": ["arch/arm/file.cpp"],
         "//conditions:default": [],
     }),
 )`,