linker: say 2MiB in more readable fashion.

...rather than require a comment to explain what we just said.

Change-Id: I519a60c1e9415ad82ec612b81a87484dc78b6ebd
diff --git a/linker/linker_phdr.cpp b/linker/linker_phdr.cpp
index e5369ac..ec2d27f 100644
--- a/linker/linker_phdr.cpp
+++ b/linker/linker_phdr.cpp
@@ -628,7 +628,7 @@
   // Minimum alignment of shared library gap. For efficiency, this should match the second level
   // page size of the platform.
 #if defined(__LP64__)
-  constexpr size_t kGapAlignment = 1ul << 21;  // 2MB
+  constexpr size_t kGapAlignment = 2 * 1024 * 1024;
 #else
   constexpr size_t kGapAlignment = 0;
 #endif