Fix stack pointer alignment for x86_64 platforms

The x86_64 ABI requires the stack pointer to be 16-byte aligned before the
call of a function and decreased by 8 at the function entrypoint (after
the return address has been pushed to the stack).

Currently, when a new Genode thread gets created, the initial stack
pointer is aligned to 16 byte. On Genode/Linux, the thread entry function
is entered by a 'call' instruction, so the stack pointer alignment at the
function entrypoint is correct. On Fiasco.OC and NOVA, however, the thread
entry function gets executed without a return address being pushed to the
stack, so at the function entrypoint the stack pointer is still aligned to
16 byte, which can cause problems with compiler-generated SSE
instructions.

With this patch, the stack pointer given to a new thread gets aligned to
16 bytes and decreased by 8 by default, since most of the currently
supported base platforms execute the thread entry function without pushing
a return address to the stack. For base-linux, the stack pointer gets
realigned to 16 bytes before the thread entry function gets called.

Fixes #1043.
Qt5: decrease memory amount needed for QtQWebkit

On 64-bit platforms Qt's JavaScript engine tries to reserve 1GiB of
virtual memory via 'mmap()', to be backed by physical memory on demand.
Genode's 'mmap()' implementation currently does not support on-demand
allocation of physical memory and tries to allocate the whole amount at
once, which is usually far more than needed.

With this patch, the amount to be reserved gets decreased to 32MiB.

Fixes #1041.