errno: implement _get_errno() in common code

We really should have more faith in the compiler, it generates
code to implement this exactly like the arch-specific assembly
versions, and on ARM is actually 4 bytes shorter.

FUNC_NO_FP used to disable the usual C preamble to update the
frame/stack pointers, which is how the sizes are still the same
or less. It's debatable how useful the occasional use of
FUNC_NO_FP is in practice since it hinders debugging and in a
production build frame pointers should be globally disabled, but
we can address that later.

Change-Id: I6c4b64ab3e3a9b6f91d52fa8c92e6e79a986fc77
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>