Move aligned allocation to the platform files.

src/platform-linux.cc

Index: src/platform-linux.cc
diff --git a/src/platform-linux.cc b/src/platform-linux.cc
index 00852baeebf66cd3be61a51ee10ade941bc98603..da5d81bd45fae5001a2bf7074bc38cc3a7e490ba 100644
--- a/src/platform-linux.cc
+++ b/src/platform-linux.cc
@@ -650,6 +650,34 @@ void* VirtualMemory::ReserveRegion(size_t size) {
 }
 
 
+void* VirtualMemory::ReserveAlignedRegion(size_t size, size_t alignment) {
+  ASSERT(IsAligned(alignment, static_cast<intptr_t>(OS::AllocateAlignment())));
+  size_t request_size = RoundUp(size + alignment,
+                                static_cast<intptr_t>(OS::AllocateAlignment()));
+  void* reservation = mmap(GetRandomMmapAddr(),
+                           request_size,
+                           PROT_NONE,
+                           MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE,
+                           kMmapFd,
+                           kMmapFdOffset);
+  if (reservation == MAP_FAILED) return NULL;
+  Address base = static_cast<Address>(reservation);
+  Address aligned_base = RoundUp(base, alignment);
+  ASSERT(base <= aligned_base);
+
+  // Unmap extra memory reserved before and after the desired block.
+  size_t bytes_prior = static_cast<size_t>(aligned_base - base);
+  if (bytes_prior > 0) {
+    munmap(base, bytes_prior);
+  }
+  if (static_cast<size_t>(aligned_base - base) < request_size - size) {
+    munmap(aligned_base + size, request_size - size - bytes_prior);
+  }
+
+  return static_cast<void*>(aligned_base);
+}
+
+
 bool VirtualMemory::CommitRegion(void* base, size_t size, bool is_executable) {
   int prot = PROT_READ | PROT_WRITE | (is_executable ? PROT_EXEC : 0);
   if (MAP_FAILED == mmap(base,