rts/linker: Align bssSize to page size when mapping symbol extras

We place symbol_extras right after bss. We also need
to ensure that symbol_extras can be mprotect'd independently from the
rest of the image. To ensure this we round up the size of bss to a page
boundary, thus ensuring that symbol_extras is also page-aligned.

(cherry picked from commit 9f40cf6c)
(cherry picked from commit 4b83b6a8)

rts/linker/SymbolExtras.c

@@ -77,7 +77,9 @@ int ocAllocateExtras(ObjectCode* oc, int count, int first, int bssSize)
       /* N.B. We currently can't mark symbol extras as non-executable in this
        * case. */
       size_t n = roundUpToPage(oc->fileSize);
-      bssSize = roundUpToAlign(bssSize, 8);
+      // round bssSize up to the nearest page size since we need to ensure that
+      // symbol_extras is aligned to a page boundary so it can be mprotect'd.
+      bssSize = roundUpToPage(bssSize);
       size_t allocated_size = n + bssSize + extras_size;
       void *new = mmapForLinker(allocated_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS, -1, 0);
       if (new) {