os: implement process related functions

This commit implements various process related functions like
os.Getuid() and os.Getpid(). It also implements or improves this support
in the syscall package if it isn't available yet.

src/syscall/proc_emulated.go

@@ -0,0 +1,13 @@
+// +build baremetal wasi wasm
+
+// This file emulates some process-related functions that are only available
+// under a real operating system.
+
+package syscall
+
+func Getuid() int  { return -1 }
+func Geteuid() int { return -1 }
+func Getgid() int  { return -1 }
+func Getegid() int { return -1 }
+func Getpid() int  { return -1 }
+func Getppid() int { return -1 }