[gold] Merging string literals with bigger alignment

[Cary Coutant]

> Hi, I implemented (b2) as we discussed above. Could you please take a look.

+  // We assume here that the beginning of the section is correctly
+  // aligned, so each string within the section must retain the same
+  // modulo.
+  uint64_t init_align_modulo = (uint64_t) pdata % this->addralign();

The type should be uintptr_t, and the cast should be
reinterpret_cast<uintptr_t>(pdata):

  uintptr_t init_align_modulo = (reinterpret_cast<uintptr_t>(pdata)
                                 & (this->addralign() - 1));

+         // Within merge input section each string must be aligned.
+         if ((uint64_t) p % this->addralign() != init_align_modulo)
+           has_misaligned_strings = true;

With a variable operand, the % operator is quite a bit slower than
masking. Please rewrite this as

          if ((reinterpret_cast<uintptr_t>(p) & (this->addralign() - 1))
              != init_align_modulo)
            has_misaligned_strings = true;

Aside from those changes, this looks good to me. Thanks!

(You still need Ian's approval to commit.)

-cary