Support grouping static libraries

LLVM has complicated static library layering that sometimes changes.
The make solution was to list all the static libraries twice, but
Soong dedups the list.  Add a group_static_libs flag to allow
surrounding the static libs with -Wl,--start-group and
-Wl,--end-group.

Test: mmma -j external/llvm
Change-Id: Ic08a183d7def9c9249d4a3014760759f16b68d04

cc/linker.go

@@ -76,6 +76,10 @@ type BaseLinkerProperties struct {
 	// don't link in crt_begin and crt_end.  This flag should only be necessary for
 	// compiling crt or libc.
 	Nocrt *bool `android:"arch_variant"`
+
+	// group static libraries.  This can resolve missing symbols issues with interdependencies
+	// between static libraries, but it is generally better to order them correctly instead.
+	Group_static_libs *bool `android:"arch_variant"`
 }
 
 func NewBaseLinker() *baseLinker {
@@ -193,6 +197,10 @@ func (linker *baseLinker) linkerFlags(ctx ModuleContext, flags Flags) Flags {
 		flags.LdFlags = append(flags.LdFlags, toolchain.ToolchainLdflags())
 	}
 
+	if Bool(linker.Properties.Group_static_libs) {
+		flags.GroupStaticLibs = true
+	}
+
 	return flags
 }