win toolchain: Always use ; for all env vars except PATH, even on non-Windows.
win toolchain: Always use ; for all env vars except PATH, even on non-Windows.

lld-link expects LIB to be ;-separated even on non-Windows.  Hopefully
it'll eventually use a flag for system lib dirs instead of looking at env vars,
but for now it looks at env vars.

(clang-cl also expects INCLUDE to be ;-separated, but it already uses
-imsvc instead of the INCLUDE env var to find system include dirs.)

BUG=495204

Review-Url: https://codereview.chromium.org/1970443002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#392676}

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