[build] Fix building on mildly deranged versions of binutils

Some versions of binutils have curious concepts of what constitutes
subtraction.  For example:

  0x00000000000000f0 _text16_late = .
  0x0000000000000898 _mtext16 = .
  0x0000000000000898 _etext16 = .
  0x0000000000000898 _text16_late_filesz = ABSOLUTE ((_mtext16 - _text16_late))
  0x00000000000007a8 _text16_late_memsz = ABSOLUTE ((_etext16 - _text16_late))

This has interesting side-effects such as producing sizes for .bss
segments that are negative, causing the majority of addressable memory
to be zeroed out.

Fix by using the form

  ABSOLUTE ( x ) - ABSOLUTE ( y )

rather than

  ABSOLUTE ( x - y )

Reported-by: H. Peter Anvin <hpa@zytor.com>
Suggested-by: H. Peter Anvin <hpa@zytor.com>
Tested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/scripts/i386.lds

     } .bss.prefix (NOLOAD) : AT ( _end_lma ) {
 	_eprefix = .;
     }
-    _prefix_filesz	= ABSOLUTE ( _mprefix - _prefix );
-    _prefix_memsz	= ABSOLUTE ( _eprefix - _prefix );
+    _prefix_filesz	= ABSOLUTE ( _mprefix ) - ABSOLUTE ( _prefix );
+    _prefix_memsz	= ABSOLUTE ( _eprefix ) - ABSOLUTE ( _prefix );
 
     /*
      * The 16-bit (real-mode) code section
     } .bss.text16 (NOLOAD) : AT ( _end_lma ) {
 	_etext16 = .;
     }
-    _text16_early_filesz = ABSOLUTE ( _etext16_early - _text16 );
-    _text16_early_memsz	= ABSOLUTE ( _etext16_early - _text16 );
-    _text16_late_filesz	= ABSOLUTE ( _mtext16 - _text16_late );
-    _text16_late_memsz	= ABSOLUTE ( _etext16 - _text16_late );
-    _text16_memsz	= ABSOLUTE ( _etext16 - _text16 );
+    _text16_early_filesz = ABSOLUTE ( _etext16_early ) - ABSOLUTE ( _text16 );
+    _text16_early_memsz	= ABSOLUTE ( _etext16_early ) - ABSOLUTE ( _text16 );
+    _text16_late_filesz	= ABSOLUTE ( _mtext16 ) - ABSOLUTE ( _text16_late );
+    _text16_late_memsz	= ABSOLUTE ( _etext16 ) - ABSOLUTE ( _text16_late );
+    _text16_memsz	= ABSOLUTE ( _etext16 ) - ABSOLUTE ( _text16 );
 
     /*
      * The 16-bit (real-mode) data section
 	*(.stack16.*)
 	_edata16 = .;
     }
-    _data16_filesz	= ABSOLUTE ( _mdata16 - _data16 );
-    _data16_memsz	= ABSOLUTE ( _edata16 - _data16 );
+    _data16_filesz	= ABSOLUTE ( _mdata16 ) - ABSOLUTE ( _data16 );
+    _data16_memsz	= ABSOLUTE ( _edata16 ) - ABSOLUTE ( _data16 );
 
     /*
      * The 32-bit sections
 	*(.stack.*)
 	_etextdata = .;
     }
-    _textdata_filesz	= ABSOLUTE ( _mtextdata - _textdata );
-    _textdata_memsz	= ABSOLUTE ( _etextdata - _textdata );
+    _textdata_filesz	= ABSOLUTE ( _mtextdata ) - ABSOLUTE ( _textdata );
+    _textdata_memsz	= ABSOLUTE ( _etextdata ) - ABSOLUTE ( _textdata );
 
     /*
      * Compressor information block
     } .bss.zinfo (NOLOAD) : AT ( _end_lma ) {
 	_ezinfo = .;
     }
-    _zinfo_filesz	= ABSOLUTE ( _mzinfo - _zinfo );
-    _zinfo_memsz	= ABSOLUTE ( _ezinfo - _zinfo );
+    _zinfo_filesz	= ABSOLUTE ( _mzinfo ) - ABSOLUTE ( _zinfo );
+    _zinfo_memsz	= ABSOLUTE ( _ezinfo ) - ABSOLUTE ( _zinfo );
 
     /*
      * Weak symbols that need zero values if not otherwise defined