acpi: fix uninitialized members of element object

The default constructor didn't initialize all members, some of them holding
pointers. In the de-constructor the _name pointer was tried to free up, even
when it was not initialized.

Avoid any hassle for uninitialized members and just initialize it. Fixes
sporadic page fault on x86_64 base-nova.

Issue #155
acpi: use xsdt on 64bit if available

Fix #155
lx_hybrid: reserve thread context area for core

core does not use POSIX threads when built for the 'lx_hybrid_x86'
platform, so we need to reserve the thread-context area via a segment in
the program to prevent clashes with vdso and shared libraries.

Fixes #639.