gyp-win-tool: Let LinkWrapper only transmogrify / when the host platform is Windows.
gyp-win-tool: Let LinkWrapper only transmogrify / when the host platform is Windows.

BUG=495204
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/1888503002 .

msc-/gyp,bnoordhuis/gyp,msc-/gyp,Phuehvk/gyp,bnoordhuis/gyp,adblockplus/gyp,Phuehvk/gyp,chromium/gyp,bnoordhuis/gyp,bnoordhuis/gyp,adblockplus/gyp,Phuehvk/gyp,msc-/gyp,adblockplus/gyp,adblockplus/gyp,chromium/gyp,adblockplus/gyp,turbulenz/gyp,turbulenz/gyp,bnoordhuis/gyp,chromium/gyp,turbulenz/gyp,turbulenz/gyp,chromium/gyp,Phuehvk/gyp,chromium/gyp,turbulenz/gyp,msc-/gyp,Phuehvk/gyp,msc-/gyp