Subzero: Add the "llvm2ice -ffunction-sections" argument.

Subzero: Add the "llvm2ice -ffunction-sections" argument.

The purpose is to enable bisection debugging of Subzero-translated functions, using objcopy to selectively splice functions from llc and Subzero into the binary.

Note that llvm-mc claims to take this argument, but actually does nothing with it, so we need to implement it in Subzero.

Also moves the ClFlags object into the GlobalContext so everyone can access it.

BUG= none
R=wala@chromium.org

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=989a703

[wala, 2014-08-08 16:42:54]

lgtm

[Jim Stichnoth, 2014-08-08 17:13:48]

Committed patchset #3 manually as r989a703 (presubmit successful).