Cherry-pick r234714: [MC] Write padding into fragments when -mc-relax-all flag is used

Cherry-pick r234714: [MC] Write padding into fragments when -mc-relax-all flag is used

Summary:
When instruction bundling is enabled and the -mc-relax-all flag is
set, we can write bundle padding directly into fragments and avoid
creating large number of fragments significantly reducing LLVM MC
memory usage.

Test Plan: Regression test attached

Reviewers: eliben

Subscribers: jfb, mseaborn

Differential Revision: http://reviews.llvm.org/D8072

BUG=https://code.google.com/p/nativeclient/issues/detail?id=4063
TEST=toolchain trybots
R=dschuff@chromium.org

Committed: https://chromium.googlesource.com/native_client/pnacl-llvm/+/d0089f0b008e03cfd141f05c80e3b628c2df75c1

[Mark Seaborn, 2015-05-12 22:33:19]

mseaborn@chromium.org changed reviewers:
+ dschuff@chromium.org

[Derek Schuff, 2015-05-12 23:01:57]

lgtm

[Mark Seaborn, 2015-05-13 01:49:17]

Committed patchset #2 (id:20001) manually as
d0089f0b008e03cfd141f05c80e3b628c2df75c1 (presubmit successful).