Strip libgcc to only keep fallback symbols

We use libgcc as fallback for symbols not present in libclang_rt
builtins, however we didn't know what exact symbols were being used,
some may not be intended to fallback.

Create libgcc_stripped, which only contains unwind symbols from libgcc.

Bug: 29275768
Test: bionic-unit-tests
Change-Id: I5b349fa6138e51663bf3b67109b880b4356da8e8

cc/builder.go

@@ -255,6 +255,7 @@ type builderFlags struct {
 	groupStaticLibs bool
 
 	stripKeepSymbols       bool
+	stripKeepSymbolsList   string
 	stripKeepMiniDebugInfo bool
 	stripAddGnuDebuglink   bool
 	stripUseGnuStrip       bool
@@ -835,6 +836,9 @@ func TransformStrip(ctx android.ModuleContext, inputFile android.Path,
 	if flags.stripKeepSymbols {
 		args += " --keep-symbols"
 	}
+	if flags.stripKeepSymbolsList != "" {
+		args += " -k" + flags.stripKeepSymbolsList
+	}
 	if flags.stripUseGnuStrip {
 		args += " --use-gnu-strip"
 	}