[prefix] Pad .text16 and .data16 segment sizes at build time

Commit c64747d ("[librm] Speed up real-to-protected mode transition
under KVM") rounded down the .text16 segment address calculated in
alloc_basemem() to a multiple of 64 bytes in order to speed up mode
transitions under KVM.

This creates a potential discrepancy between alloc_basemem() and
free_basemem(), meaning that free_basemem() may free less memory than
was allocated by alloc_basemem().

Fix by padding the calculated sizes of both .text16 and .data16 to a
multiple of 64 bytes at build time.

Debugged-by: Yossef Efraim <yossefe@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/libprefix.S

 	shlw	$6, %ax
 
 	/* Calculate .data16 segment address */
-	subw	$_data16_memsz_pgh, %ax
+	subw	$_data16_memsz_ppgh, %ax
 	pushw	%ax
 
-	/* Calculate .text16 segment address.  Round down to ensure
-	 * low bits are zero, to speed up mode transitions under KVM.
-	 */
-	subw	$_text16_memsz_pgh, %ax
-	andb	$~0x03, %al
+	/* Calculate .text16 segment address */
+	subw	$_text16_memsz_ppgh, %ax
 	pushw	%ax
 
 	/* Update FBMS */
 
 	/* OK to free memory */
 	movw	%cs, %ax
-	addw	$_text16_memsz_pgh, %ax
-	addw	$_data16_memsz_pgh, %ax
+	addw	$_text16_memsz_ppgh, %ax
+	addw	$_data16_memsz_ppgh, %ax
 	shrw	$6, %ax
 	movw	%ax, %fs:0x13
 	xorw	%ax, %ax

src/arch/i386/scripts/i386.lds

      * Values calculated to save code from doing it
      *
      */
-    _text16_memsz_pgh	= ( ( _text16_memsz + 15 ) / 16 );
-    _data16_memsz_pgh	= ( ( _data16_memsz + 15 ) / 16 );
+    _text16_memsz_ppgh	= ( ( ( _text16_memsz + 63 ) / 64 ) * 4 );
+    _data16_memsz_ppgh	= ( ( ( _data16_memsz + 63 ) / 64 ) * 4 );
     _textdata_memsz_pgh	= ( ( _textdata_memsz + 15 ) / 16 );
     _textdata_memsz_kb	= ( ( _textdata_memsz + 1023 ) / 1024 );
 }