Only use 64bit atomics when supported

To support Windows XP we must avoid using the 64bit variants of the
atomicIncrement/atomicDecrement operations which are only supported from
Windows 2003 and up.

This patch rolls back the minimum Win32 API version to WinNT 5.1 (32bit WinXP),
and adds a new flag to indicate the support of 64bit atomics.

The new flag is now also used to support MIPS and other architectures
without 64bit intrinsics, instead of hacking in poor support.

This also extends the atomic operations to sparc64 which was previously
skipped because it did not work with __exchange_and_add.

Task-number: QTBUG-34271
Change-Id: I21b09df3cafb7f0987a2f44c89036ff34ed34aa0
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Redisable debug-info on MinGW64

Change f6069ba2adac13eee941a300595aa3c0a445fc1a disabled the stripping
of debug information from linux x64 builds, but accidentally also
disabled it for MinGW64 builds.

This patch ensures we again strip the deeper WebKit debug information
from all win32-g++ builds

Change-Id: I06e2a85f71cb159205a0e9acae01806ecf4e6b0f
Reviewed-by: Jonathan Liu <net147@gmail.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>