[librm] Provide symbols for inline code placed into other sections

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>

src/arch/x86/include/librm.h

 #define CODE_DEFAULT ".code32"
 #endif
 
+/* LINE_SYMBOL: declare a symbol for the current source code line */
+#define LINE_SYMBOL _S2 ( OBJECT ) "__line_" _S2 ( __LINE__ ) ":"
+
 /* TEXT16_CODE: declare a fragment of code that resides in .text16 */
 #define TEXT16_CODE( asm_code_str )			\
 	".section \".text16\", \"ax\", @progbits\n\t"	\
+	"\n" LINE_SYMBOL "\n\t"				\
 	".code16\n\t"					\
 	asm_code_str "\n\t"				\
 	CODE_DEFAULT "\n\t"				\
 	"push $1f\n\t"					\
 	"call phys_call\n\t"				\
 	".section \".text.phys\", \"ax\", @progbits\n\t"\
+	"\n" LINE_SYMBOL "\n\t"				\
 	".code32\n\t"					\
 	"\n1:\n\t"					\
 	asm_code_str					\