vmbase_example: Clarify that the payload is a BIOS

Rename all Soong targets to include "_bios" and refactor the linker
script and idmap.S to be preprocessed and receive a CPP macro
representing the "mode". Rename image.ld to reflect this.

Modify the test to use the new "_bios" binary name and refactor it to
make the core logic re-usable for future vmbase_example modes.

No functional change intended.

Test: atest vmbase_example.integration_test
Change-Id: I06679d154494d85916ff534298a066519b1b0778
diff --git a/guest/vmbase_example/Android.bp b/guest/vmbase_example/Android.bp
index 630cee6..86a63b7 100644
--- a/guest/vmbase_example/Android.bp
+++ b/guest/vmbase_example/Android.bp
@@ -19,8 +19,41 @@
     ],
 }
 
-cc_binary {
-    name: "vmbase_example",
+genrule {
+    name: "vmbase_image.ld.S.mm",
+    // Soong won't let us use cc_object to preprocess *.ld.S files because it
+    // can't resist feeding any and all *.S files to the assembler, which fails
+    // because linker scripts typically aren't valid assembly. Also, cc_object
+    // rejects inputs that don't end in one of .{s,S,c,cpp,cc,cxx,mm}. So keep
+    // the proper extension (.ld.S) for the file in VCS and use this convoluted
+    // extra step to please Soong by pretending that our linker script is in
+    // fact some Object C++ code, which fortunately it doesn't try to compile.
+    srcs: ["image.ld.S"],
+    out: ["image.ld.S.mm"],
+    cmd: "cp $(in) $(out)",
+    visibility: ["//visibility:private"],
+}
+
+cc_defaults {
+    name: "vmbase_example_ld_defaults",
+    defaults: ["vmbase_cc_defaults"],
+    cflags: [
+        "-E",
+        "-P",
+        "-xassembler-with-cpp", // allow C preprocessor directives
+    ],
+    srcs: [":vmbase_image.ld.S.mm"],
+    visibility: ["//visibility:private"],
+}
+
+cc_object {
+    name: "vmbase_example_bios.ld",
+    defaults: ["vmbase_example_ld_defaults"],
+    cflags: ["-DVMBASE_EXAMPLE_IS_BIOS"],
+}
+
+cc_defaults {
+    name: "vmbase_example_elf_defaults",
     defaults: ["vmbase_elf_defaults"],
     srcs: [
         "idmap.S",
@@ -28,16 +61,22 @@
     static_libs: [
         "libvmbase_example",
     ],
+}
+
+cc_binary {
+    name: "vmbase_example_bios",
+    defaults: ["vmbase_example_elf_defaults"],
+    asflags: ["-DVMBASE_EXAMPLE_IS_BIOS"],
     linker_scripts: [
-        "image.ld",
+        ":vmbase_example_bios.ld",
         ":vmbase_sections",
     ],
 }
 
 raw_binary {
-    name: "vmbase_example_bin",
-    stem: "vmbase_example.bin",
-    src: ":vmbase_example",
+    name: "vmbase_example_bios_bin",
+    stem: "vmbase_example_bios.bin",
+    src: ":vmbase_example_bios",
     enabled: false,
     target: {
         android_arm64: {