Use -static-libgcc for Windows

Bug: http://b/69970955

With the MinGW prebuilts update (http://aosp/709402), executables get a
runtime dependency on libgcc_s_sjlj-1.dll with both gcc and Clang.
Passing -static-libgcc removes this dependency and is a better option
than having to package an extra dll with executables.

Test: Run gcc-built adb_test.exe under wine
Change-Id: I18e5fadb1b8ace510ae684e2d4cfa7fe7d007cff