syscall: stub mmap(), munmap(), MAP_SHARED, PROT_READ, SIGBUS, etc. on nonhosted targets

Makes 1.18 tests a little happier.

Works around this error:

$ make test GOTESTFLAGS="-run TestTest/EmulatedCortexM3/Pass"
...
main_test.go:520: test error: could not compile: /usr/local/go/src/internal/fuzz/sys_posix.go:19:18: PROT_READ not declared by package syscall

src/syscall/syscall_nonhosted.go

@@ -27,6 +27,12 @@ const (
 	SIGTRAP
 	SIGQUIT
 	SIGTERM
+	SIGILL
+	SIGABRT
+	SIGBUS
+	SIGFPE
+	SIGSEGV
+	SIGPIPE
 )
 
 func (s Signal) Signal() {}
@@ -66,6 +72,22 @@ const (
 	O_CLOEXEC = 0
 )
 
+// Dummy values to allow compiling tests
+// Dummy source: https://opensource.apple.com/source/xnu/xnu-7195.81.3/bsd/sys/mman.h.auto.html
+const (
+	PROT_NONE  = 0x00 // no permissions
+	PROT_READ  = 0x01 // pages can be read
+	PROT_WRITE = 0x02 // pages can be written
+	PROT_EXEC  = 0x04 // pages can be executed
+
+	MAP_SHARED  = 0x0001 // share changes
+	MAP_PRIVATE = 0x0002 // changes are private
+
+	MAP_FILE      = 0x0000 // map from file (default)
+	MAP_ANON      = 0x1000 // allocated from memory, swap space
+	MAP_ANONYMOUS = MAP_ANON
+)
+
 func runtime_envs() []string
 
 func Getenv(key string) (value string, found bool) {
@@ -168,6 +190,14 @@ func Wait4(pid int, wstatus *WaitStatus, options int, rusage *Rusage) (wpid int,
 	return 0, ENOSYS
 }
 
+func Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, err error) {
+	return nil, ENOSYS
+}
+
+func Munmap(b []byte) (err error) {
+	return ENOSYS
+}
+
 type Timeval struct {
 	Sec  int64
 	Usec int64