Mark start/stop sections as SHF_GNU_RETAIN

Fixes swiftlang/swift-package-manager#5698

Author: keith

stdlib/public/runtime/SwiftRT-ELF.cpp

@@ -23,7 +23,7 @@ extern "C" const char __dso_handle[];
 // the linker table section was never constructed.
 
 #define DECLARE_SWIFT_SECTION(name)                                                          \
-  __asm__("\t.section " #name ",\"a\"\n");                                                   \
+  __asm__("\t.section " #name ",\"aR\"\n");                                                   \
   __attribute__((__visibility__("hidden"),__aligned__(1))) extern const char __start_##name; \
   __attribute__((__visibility__("hidden"),__aligned__(1))) extern const char __stop_##name;