[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/tests/cms_test.c

@@ -1474,6 +1474,7 @@ struct self_test cms_test __self_test = {
 };
 
 /* Drag in algorithms required for tests */
+REQUIRING_SYMBOL ( cms_test );
 REQUIRE_OBJECT ( rsa );
 REQUIRE_OBJECT ( md5 );
 REQUIRE_OBJECT ( sha1 );

src/tests/ocsp_test.c

@@ -1861,5 +1861,6 @@ struct self_test ocsp_test __self_test = {
 };
 
 /* Drag in algorithms required for tests */
+REQUIRING_SYMBOL ( ocsp_test );
 REQUIRE_OBJECT ( rsa );
 REQUIRE_OBJECT ( sha1 );

src/tests/tests.c

@@ -30,6 +30,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  */
 
 /* Drag in all applicable self-tests */
+PROVIDE_REQUIRING_SYMBOL();
 REQUIRE_OBJECT ( memset_test );
 REQUIRE_OBJECT ( memcpy_test );
 REQUIRE_OBJECT ( string_test );

src/tests/x509_test.c

@@ -1109,6 +1109,7 @@ struct self_test x509_test __self_test = {
 };
 
 /* Drag in algorithms required for tests */
+REQUIRING_SYMBOL ( x509_test );
 REQUIRE_OBJECT ( rsa );
 REQUIRE_OBJECT ( sha1 );
 REQUIRE_OBJECT ( sha256 );