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_library_conversion_test.go b/bp2build/cc_library_conversion_test.go
index 285677a..4f720f5 100644
--- a/bp2build/cc_library_conversion_test.go
+++ b/bp2build/cc_library_conversion_test.go
@@ -674,6 +674,10 @@
 		blueprint: soongCcLibraryPreamble,
 		expectedBazelTargets: []string{`cc_library(
     name = "a",
+    asflags = [
+        "-Ifoo/bar",
+        "-I$(BINDIR)/foo/bar",
+    ],
     copts = [
         "-Ifoo/bar",
         "-I$(BINDIR)/foo/bar",