Fix hard coded values in dispatch_sysall inline assembly. Back out/corect part of last change; that was going the wrong direction.

Fix ARM7/9 and Cortex-A SYSCALLs:  For threads in SVC mode, the SVC instructions clobbers R14.  This must be taken account in the inline assembly

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

Trick the top-level makefiles into building the SYSCALL proxy library but then not actually linking with it

include/sys/syscall.h: System definitions should depend on CONFIG_LIB_SYSCALL not CONFIG_NUTTX_KERNEL

nuttx/sched:  Remove explicit references to errno.  That is a problem from within the kernel for certain configurations

nuttx/sched:  Remove explicit references to errno.  That is a problem from within the kernel for certain configurations

nuttx/drivers:  Remove explicity references to errno.  That is a problem from within the kernel for certain configurations

nuttx/fs:  Remove explicity references to errno.  That is a problem from within the kernel for certain configurations

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