Rename  arch/arm/src/armv7-a/syscall.h to svcall.h to work around some include path name collisions; fix some compilation errors in SYSCALL logic when debug is enabled

STM32 FLASH fixes: use size_t instead of uint16_t, make interface more generic.  From Freddie Chopin

Z80: Move address environment switch from the task switchers to the interrupt handler.  That may save doing the action multiple times per interrupt

x86: Move address environment switch from the task switchers to the interrupt handler.  That may save doing the action multiple times per interrupt

SH: Move address environment switch from the task switchers to the interrupt handler.  That may save doing the action multiple times per interrupt

MIPS: Move address environment switch from the task switchers to the interrupt handler.  That may save doing the action multiple times per interrupt

AVR32: Move address environment switch from the task switchers to the interrupt handler.  That may save doing the action multiple times per interrupt

AVR32: Move address environment switch from the task switchers to the interrupt handler.  That may save doing the actin multiple times per interrupt

ARM: Move address environment switch from the task switchers to the interrupt handler.  That may save doing the actin multiple times per interrupt

ARM: Move address environment switch from the task switchers to the interrupt handler.  That may save doing the actin multiple times per interrupt

Rename up_addrenv_assign() to up_addrenv_clone() and generalize its arguments so that can be used for other purposes

Fix confusion about what is a page of data and what is a page of L2 page table; restructure functions to reduce duplicated logic