Extend wasi-emulated-mman with mprotect. (WebAssembly#500)

This implementation never changes actual memory protection, but it does
verify that the address range and flags are valid. The direct motivation
is fixing a linker error where LLVM links to `mprotect` in dead code.

expected/wasm32-wasip1-threads/defined-symbols.txt

@@ -928,6 +928,7 @@ mmap
 modf
 modff
 modfl
+mprotect
 mrand48
 munmap
 nan

expected/wasm32-wasip1/defined-symbols.txt

@@ -861,6 +861,7 @@ mmap
 modf
 modff
 modfl
+mprotect
 mrand48
 munmap
 nan

expected/wasm32-wasip2/defined-symbols.txt

@@ -980,6 +980,7 @@ monotonic_clock_now
 monotonic_clock_resolution
 monotonic_clock_subscribe_duration
 monotonic_clock_subscribe_instant
+mprotect
 mrand48
 munmap
 nan

libc-bottom-half/mman/mman.c

@@ -9,6 +9,7 @@
 #include <errno.h>
 #include <unistd.h>
 #include <string.h>
+#include <limits.h>
 #include <sys/mman.h>
 #include <sys/types.h>
 
@@ -122,3 +123,35 @@ int munmap(void *addr, size_t length) {
     // Success!
     return 0;
 }
+
+int mprotect(void *addr, size_t length, int prot) {
+    // Address must be page-aligned.
+    size_t begin = (size_t)addr;
+    if ((begin & (PAGESIZE - 1)) != 0) {
+        errno = EINVAL;
+        return -1;
+    }
+
+    // Length must not be big enough to wrap around.
+    size_t end;
+    if (__builtin_add_overflow(begin, length, &end)) {
+        errno = ENOMEM;
+        return -1;
+    }
+
+    // Range must be in bounds of linear memory.
+    size_t memory_size = __builtin_wasm_memory_size(0) * PAGESIZE;
+    if (end > memory_size) {
+        errno = ENOMEM;
+        return -1;
+    }
+
+    // Can only protect memory as read/write (which is a no-op).
+    if (prot != (PROT_READ | PROT_WRITE)) {
+        errno = ENOTSUP;
+        return -1;
+    }
+
+    // Success!
+    return 0;
+}