Support system's LDFLAGS, make compilation reproducible

[maximbaz]

• Supporting $LDFLAGS provided by the build system for example allows enabling RELRO.
• Using -trimpath in -gcflags and -asmflags makes your application reproducible.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

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[codecov-io]

Codecov Report

Merging #1270 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1270   +/-   ##
=======================================
  Coverage   42.42%   42.42%           
=======================================
  Files          96       96           
  Lines        4247     4247           
=======================================
  Hits         1802     1802           
  Misses       2269     2269           
  Partials      176      176

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[maximbaz]

Signed CLA

[googlebot]

CLAs look good, thanks!

[balopat]

LGTM