[linker] Expand and correct symbol requirement macros

REQUIRE_SYMBOL() formerly used a formulation of symbol requirement
that would allow a link to succeed despite lacking a required symbol,
because it did not introduce any relocations. Fix by renaming it to
REQUEST_SYMBOL() (since the soft-requirement behavior can be useful)
and add a REQUIRE_SYMBOL() that truly requires.

Add EXPORT_SYMBOL() and IMPORT_SYMBOL() for REQUEST_SYMBOL()-like
behavior that allows one to make use of the symbol, by combining a
weak external on the symbol itself with a REQUEST_SYMBOL() of a second
symbol.

Signed-off-by: Marty Connor <mdc@etherboot.org>

src/arch/i386/scripts/i386-kir.lds

@@ -130,6 +130,7 @@ SECTIONS {
     /DISCARD/ : {
 	*(.comment)
 	*(.note)
+	*(.discard)
     }
 
     /*

src/arch/i386/scripts/i386.lds

@@ -152,6 +152,7 @@ SECTIONS {
 	*(.eh_frame.*)
 	*(.rel)
 	*(.rel.*)
+	*(.discard)
     }
 
     /*

src/arch/x86/scripts/efi.lds

@@ -101,5 +101,6 @@ SECTIONS {
 	*(.eh_frame.*)
 	*(.rel)
 	*(.rel.*)
+	*(.discard)
     }
 }