Provide symbols constructed from the object name and line number for
code fragments placed into alternative sections, such as inline
REAL_CODE() assembly placed into .text16.  This simplifies the
debugging task of finding the source code corresponding to a given
instruction pointer.

Note that we cannot use __FUNCTION__ since it is not a preprocessor
macro and so cannot be concatenated with string literals.

Signed-off-by: Michael Brown <mcb30@ipxe.org>