arm64: stacktrace: Move start_backtrace() out of the header

arm64: stacktrace: Move start_backtrace() out of the header

Currently start_backtrace() is a static inline function in the header.
Since it really shouldn't be sufficiently performance critical that we
actually need to have it inlined move it into a C file, this will save
anyone else scratching their head about why it is defined in the header.
As far as I can see it's only there because it was factored out of the
various callers.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210319174022.33051-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>