32-bit linux: Force 16-byte stack alignment.

clang assumes 16-byte stack alignment, but incoming stack alignment isn't
always guaranteed to be that way.  It looks like v8 was lucky to not hit
this so far.

See https://crbug.com/418554 -- this makes v8's standalone config match
Chromium.  See also https://llvm.org/bugs/show_bug.cgi?id=21414

Maybe it's possible to change the caller of OnEntryHook() to guarantee
the right alignment, but matching Chromium's build flags here seems like
a good idea in general.

BUG=v8:4928
LOG=n

Committed: https://crrev.com/3afb3324941625559635380ef98a2ee73e370a0a
Cr-Commit-Position: refs/heads/master@{#35597}

Review URL: https://codereview.chromium.org/1899783002

Cr-Commit-Position: refs/heads/master@{#35662}

build/standalone.gypi

@@ -463,6 +463,9 @@
           }],
         ],
       }],
+      ['clang==1 and target_arch=="ia32"', {
+        'cflags': ['-mstack-alignment=16', '-mstackrealign'],
+      }],
       ['fastbuild!=0', {
         'conditions': [
           ['OS=="win" and fastbuild==1', {