[librm] Ensure that inline code symbols are unique

[librm] Ensure that inline code symbols are unique

Commit 6149e0a ("[librm] Provide symbols for inline code placed into
other sections") may cause build failures due to duplicate label names
if the compiler chooses to duplicate inline assembly code.

Fix by using the "%=" special format string to include a
guaranteed-unique number within the label name.

The "%=" will be expanded only if constraints exist for the inline
assembly.  This fix therefore requires that all REAL_CODE() fragments
use a (possibly empty) constraint list.

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