Instead of changing the attributes (e.g., Xd bit) of the top-level page-tables,
set them to allow everything. Only leafs of the paging hierarchy are set
according to the paging attributes given by core. Otherwise, top-level page-
table attributes are changed during lifetime, which requires a TLB flush
operation (not intended in the semantic of the kernel/core).
This led to problems when using the non-executable features introduced by
issue #1723 in the recent past.
* Instead of always re-load page-tables when a thread context is switched
only do this when another user PD's thread is the next target,
core-threads are always executed within the last PD's page-table set
* remove the concept of the mode transition
* instead map the exception vector once in bootstrap code into kernel's
memory segment
* when a new page directory is constructed for a user PD, copy over the
top-level kernel segment entries on RISCV and X86, on ARM we use a designated
page directory register for the kernel segment
* transfer the current CPU id from bootstrap to core/kernel in a register
to ease first stack address calculation
* align cpu context member of threads and vms, because of x86 constraints
regarding the stack-pointer loading
* introduce Align_at template for members with alignment constraints
* let the x86 hardware do part of the context saving in ISS, by passing
the thread context into the TSS before leaving to user-land
* use one exception vector for all ARM platforms including Arm_v6
Fix#2091
Stefan Kalkowski