[build] Work around binutils quirk on OpenBSD 5.7

The assembler on OpenBSD 5.7 seems not to correctly handle the
combinations of .struct and .previous used in unlzma.S, and ends up
complaining about an "attempt to allocate data in absolute section".

Work around this problem by explicitly resetting the section after the
data structure definitions.

Reported-by: Jiri B <jirib@devio.us>
Tested-by: Jiri B <jirib@devio.us>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/unlzma.S

@@ -232,6 +232,9 @@ literal:	.rept	( ( 1 << LZMA_LC ) * 0x300 )
 	.equ	sizeof__lzma_dec, . - lzma_dec
 	.previous
 
+	/* Some binutils versions seem not to handle .struct/.previous */
+	.section ".prefix.lib", "ax", @progbits
+
 /*****************************************************************************
  * Normalise range encoder
  *