lib: aarch64: fix non-code symbol errors flagged by armlink

This patch modifies the code to turn __1printf and __2printf into proper
functions to fix the following errors flagged by armlink.

Error: L6318W: backtrace.o(.text.backtrace) contains branch to a non-code symbol __2printf.
Error: L6318W: backtrace.o(.text.backtrace) contains branch to a non-code symbol __2printf.
Error: L6318W: backtrace.o(.text.backtrace) contains branch to a non-code symbol __2printf.
Error: L6318W: backtrace.o(.text.backtrace) contains branch to a non-code symbol __2printf.
Error: L6318W: backtrace.o(.text.backtrace) contains branch to a non-code symbol __2printf.
Error: L6318W: backtrace.o(.text.backtrace) contains branch to a non-code symbol __2printf.

Change-Id: I89126bc2b9db44ce8b8fc9fb1e3fc4c8c60c47a4
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>