[wasm] Enable --gc-sections for WebAssembly by default

This change enables the `--gc-sections` linker flag for WebAssembly
targets by default. This flag has been disabled because it did not
respect `llvm.used` and it caused stripping `swift5` metadata sections
even though they were used through encapsulation symbols (a.k.a `__start`/`__stop`).

The issue has been fixed in the LLVM side (llvm/llvm-project@ba3c1f9)
by adding new segment flags to the WebAssembly object file format.

Author: kateinoigakukun

stdlib/public/Resources/wasi/static-executable-args.lnk

@@ -7,5 +7,4 @@
 -lwasi-emulated-signal
 -lwasi-emulated-process-clocks
 -Xlinker --error-limit=0
--Xlinker --no-gc-sections
 -Xlinker --threads=1

stdlib/public/runtime/SwiftRT-ELF-WASM.cpp

@@ -38,7 +38,7 @@ static const void *__backtraceRef __attribute__((used))
 #if defined(__ELF__)
 # define DECLARE_EMPTY_METADATA_SECTION(name) __asm__("\t.section " #name ",\"a\"\n");
 #elif defined(__wasm__)
-# define DECLARE_EMPTY_METADATA_SECTION(name) __asm__("\t.section " #name ",\"\",@\n");
+# define DECLARE_EMPTY_METADATA_SECTION(name) __asm__("\t.section " #name ",\"R\",@\n");
 #endif
 
 #define DECLARE_SWIFT_SECTION(name)                                                          \