WASI & darwin: support env variables based on libc

Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>

src/syscall/syscall_libc_darwin.go

@@ -0,0 +1,56 @@
+package syscall
+
+// This file defines errno and constants to match the darwin libsystem ABI.
+// Values have been copied from src/syscall/zerrors_darwin_amd64.go.
+
+// This function returns the error location in the darwin ABI.
+// Discovered by compiling the following code using Clang:
+//
+//     #include <errno.h>
+//     int getErrno() {
+//         return errno;
+//     }
+//
+//export __error
+func libc___error() *int32
+
+// getErrno returns the current C errno. It may not have been caused by the last
+// call, so it should only be relied upon when the last call indicates an error
+// (for example, by returning -1).
+func getErrno() Errno {
+	errptr := libc___error()
+	return Errno(uintptr(*errptr))
+}
+
+const (
+	ENOENT      Errno = 0x2
+	EEXIST      Errno = 0x11
+	EINTR       Errno = 0x4
+	EMFILE      Errno = 0x18
+	EAGAIN      Errno = 0x23
+	ETIMEDOUT   Errno = 0x3c
+	ENOSYS      Errno = 0x4e
+	EWOULDBLOCK Errno = EAGAIN
+)
+
+type Signal int
+
+const (
+	SIGCHLD Signal = 0x14
+	SIGINT  Signal = 0x2
+	SIGKILL Signal = 0x9
+	SIGTRAP Signal = 0x5
+	SIGQUIT Signal = 0x3
+	SIGTERM Signal = 0xf
+)
+
+const (
+	O_RDONLY = 0x0
+	O_WRONLY = 0x1
+	O_RDWR   = 0x2
+	O_APPEND = 0x8
+	O_SYNC   = 0x80
+	O_CREAT  = 0x200
+	O_TRUNC  = 0x400
+	O_EXCL   = 0x800
+)