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Before this CL, only EXE files had CFG bitmap generated
and PE DLL Characteristics "Control Flow Guard" enabled.

This CL enables these for DLLs as well. It is a no-op for
Chrome code as the dispatch guards are not being generated
but will protect indirect calls from e.g. system libraries
from being able to call into arbitrary Chrome code.

This CL also updates checkbins.py to ensure that shipped
binaries always have /guard:CF enabled, and also enables
longjmp support as compiler issues were isolated to MSVC
which we no longer use.

See Comment 30 on bug for size impact analysis.

TEST=Verify by running dumpbin.exe on Chrome DLLs and
checking for "Control Flow Guard" in the "DLL characteristics"
header field, or run tools/checkbins/checkbins.py on
output directory.

BUG=584575

Cq-Include-Trybots: luci.chromium.try:win7-rel,win_archive,win_x64_archive
Change-Id: I4b78b97e93cd81dee70d0b3d1edd3d8e522a5495
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298284
Commit-Queue: Will Harris <wfh@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789208}