[Arm] SP and SL initialization refactored

[Alexander Fedotov]

Hi all

In continue of threads
https://sourceware.org/ml/newlib/2019/msg00117.html and
https://sourceware.org/ml/newlib/2019/msg00102.html

I have reworked stack pointer initialization and stack limit as well.
General idea behind of all these changes is to provide hook for
flexibility (including semihosting when debug tap is used) and improve
readability of existing crt0 code.
And now code makes SP initialization in Thumb mode :)

Any comments are very appreciated.

Alex
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