[build] Fix the REQUIRE_SYMBOL mechanism

At some point in the past few years, binutils became more aggressive
at removing unused symbols.  To function as a symbol requirement, a
relocation record must now be in a section marked with @progbits and
must not be in a section which gets discarded during the link (either
via --gc-sections or via /DISCARD/).

Update REQUIRE_SYMBOL() to generate relocation records meeting these
criteria.  To minimise the impact upon the final binary size, we use
existing symbols (specified via the REQUIRING_SYMBOL() macro) as the
relocation targets where possible.  We use R_386_NONE or R_X86_64_NONE
relocation types to prevent any actual unwanted relocation taking
place.  Where no suitable symbol exists for REQUIRING_SYMBOL() (such
as in config.c), the macro PROVIDE_REQUIRING_SYMBOL() can be used to
generate a one-byte-long symbol to act as the relocation target.

If there are versions of binutils for which this approach fails, then
the fallback will probably involve killing off REQUEST_SYMBOL(),
redefining REQUIRE_SYMBOL() to use the current definition of
REQUEST_SYMBOL(), and postprocessing the linked ELF file with
something along the lines of "nm -u | wc -l" to check that there are
no undefined symbols remaining.

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

src/arch/i386/include/bits/compiler.h

@@ -3,6 +3,9 @@
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
+/** Dummy relocation type */
+#define RELOC_TYPE_NONE R_386_NONE
+
 #ifndef ASSEMBLY
 
 /** Declare a function with standard calling conventions */

src/arch/i386/interface/pxe/pxe_call.c

@@ -346,6 +346,7 @@ int pxe_start_nbp ( void ) {
 	return 0;
 }
 
+REQUIRING_SYMBOL ( pxe_api_call );
 REQUIRE_OBJECT ( pxe_preboot );
 REQUIRE_OBJECT ( pxe_undi );
 REQUIRE_OBJECT ( pxe_udp );

src/arch/i386/prefix/kkkpxeprefix.S

@@ -8,6 +8,7 @@
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
 
 /* Provide the PXENV_FILE_EXIT_HOOK API call */
+REQUIRING_SYMBOL ( _kkkpxe_start )
 REQUIRE_OBJECT ( pxe_exit_hook )
 
 #define PXELOADER_KEEP_UNDI

src/arch/i386/prefix/romprefix.S

@@ -875,5 +875,8 @@ wait_for_tick:
 	ret
 	.size wait_for_tick, . - wait_for_tick
 
+/* Drag in objects via _rom_start */
+REQUIRING_SYMBOL ( _rom_start )
+
 /* Drag in ROM configuration */
 REQUIRE_OBJECT ( config_romprefix )

src/arch/i386/transitions/librm_test.c

@@ -118,4 +118,5 @@ struct self_test librm_test __self_test = {
 	.exec = librm_test_exec,
 };
 
+REQUIRING_SYMBOL ( librm_test );
 REQUIRE_OBJECT ( test );