Use fixed executable allocator on x64

Since the x86-64 assembler generator depends on jumps being expressable
as 32bit offsets we need to use the fixed size executable allocator and
not the on demand allocator. This patch removes its exclusion from being
used on Windows.

Task-number: QTBUG-42417
Change-Id: Ie6b127e9b4291869566fd99cc7efe05b566c8e66
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>