Move newlib syscall implementation outside core archive

Due to a bug in the arm none eabi gcc with lto which raise
an unresolved reference if syscall is in the archive (_sbrk,...)

See:
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka15833.html

Signed-off-by: Frederic Pillon <[email protected]>

Author: fpistm