Fix initializing the allocator before creating empty vector.

Fix initializing the allocator before creating empty vector.

MSVC's implementation of the STL allocates memory even for an empty
vector. Since we're using a custom thread-local allocator, it should
be initialized before any STL member containers get initialized.

BUG=swiftshader:7

Change-Id: I4bd977e7ee8eb87006fe08b051cbcfc9bc62342b
Reviewed-on: https://chromium-review.googlesource.com/381531
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Jim Stichnoth <stichnot@chromium.org>