Build: Remove toolchain reset mechanism

The preload.cmake and compiler/linker reset magic is not necessary in
the split build. Building S and NS sides seprartely allows setting
configuration for setting CPU and Arch explicitly.

The changes applied for an521 platforms. Builds for regression tests.

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Signed-off-by: Dávid Házi <david.hazi@arm.com>
Change-Id: I4a431eab8a3cb13af2267290805d396285f57385
16 files changed