RFR: 8304893: Link Time Optimization with gcc can be faster [v4]

[Julian Waters]

> A previous argument against link time optimization support that we have for gcc is that it was extremely slow. After some checks it turns out we are passing rather inefficient flags to gcc in optimized builds. Changing these flags to run the linker optimizations in parallel and passing additional flags to the compiler have the ability to speed this process up significantly. Also fixes some incorrect flags passed to ld for linking as well, since strict-aliasing is required for both linker and compiler, and the same optimization level should be specified instead of letting the default -O1 be passed to the linker

Julian Waters has updated the pull request incrementally with one additional commit since the last revision:

  Should auto detect cores as a safer fallback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13180/files
  - new: https://git.openjdk.org/jdk/pull/13180/files/1fe1ba77..e04abcc5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13180&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13180&range=02-03

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13180.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13180/head:pull/13180

PR: https://git.openjdk.org/jdk/pull/13180