stmhal: Initialise stack pointer correctly.
Stack is full descending and must be 8-byte aligned. It must start off pointing to just above the last byte of RAM. Previously, stack started pointed to last byte of RAM (eg 0x2001ffff) and so was not 8-byte aligned. This caused a bug in combination with alloca. This patch also updates some debug printing code. Addresses issue #872 (among many other undiscovered issues).