runtime: only initialize os.runtime_args when needed

This generally means that code size is reduced, especially when the os
package is not imported.

Specifically:

  - On Linux (which currently statically links musl), it avoids calling
    malloc, which avoids including the musl C heap for small programs
    saving around 1.6kB.
  - On WASI, it avoids initializing the args slice when the os package
    is not used. This reduces binary size by around 1kB.

src/runtime/runtime.go

@@ -23,16 +23,6 @@ func GOROOT() string {
 	return "/usr/local/go"
 }
 
-// This is the default set of arguments, if nothing else has been set.
-// This may be overriden by modifying this global at runtime init (for example,
-// on Linux where there are real command line arguments).
-var args = []string{"/proc/self/exe"}
-
-//go:linkname os_runtime_args os.runtime_args
-func os_runtime_args() []string {
-	return args
-}
-
 // Copy size bytes from src to dst. The memory areas must not overlap.
 // Calls to this function are converted to LLVM intrinsic calls such as
 // llvm.memcpy.p0i8.p0i8.i32(dst, src, size, false).