platform: posix fix open read-only file

Bug: v8:8997
Change-Id: If47a9c4d580b29f67eaadc4b129279b085e428b2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1530812
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60336}

src/base/platform/platform-posix.cc

@@ -446,7 +446,8 @@ class PosixMemoryMappedFile final : public OS::MemoryMappedFile {
 // static
 OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name,
                                                  FileMode mode) {
-  if (FILE* file = fopen(name, "r+")) {
+  const char* fopen_mode = (mode == FileMode::kReadOnly) ? "r" : "r+";
+  if (FILE* file = fopen(name, fopen_mode)) {
     if (fseek(file, 0, SEEK_END) == 0) {
       long size = ftell(file);  // NOLINT(runtime/int)
       if (size == 0) return new PosixMemoryMappedFile(file, nullptr, 0);