Sync Windows build flags between GYP and GN

Sync Windows build flags between GYP and GN

In GN, turn on omtimize:max for base targets on all platforms, not just Android (matches GYP).

In GN, don't duplicate /Os twice, make standard optimize = /O1 (matches GYP). Don't specify redundant optimization flags that are included as part of /O1 or /O2 (doesn't match GYP flag-wise, but should match functionality-wise).

In GN, add a min Windows version of XP to x86 architecture binaries (matches GYP).

In GYP, remove /GT ("EnableFiberSafeOptimizations"). This flag is super old (pre-dates GYP) and was likely turned on because it has the word "optimization" in it. It was only specified in official builds. However, according to the docs, this is making optimizations fiber safe (i.e. slower, by not caching the TLS pointer), rather than enabling optimizations for fibers. We don't use fibers so this flag is unnecessary and may be making TLS access slightly slower.

Review URL: https://codereview.chromium.org/1183613006

Cr-Commit-Position: refs/heads/master@{#334742}