asmtest: avoid testing against complex nop alignment in gas

.align #,0x90 in gas ignores the 0x90 and outputs any kind
of nop it feels like.  the one avoided by this patch is a 7
byte nop, which gas has been doing since at least 1999:
http://sourceware.org/ml/binutils/1999-10/msg00083.html

In order to match what gas does, we would need to make
code alignment target-specific, import a lot of code, and
face the question: exactly which gas {version,target,tune}
combo are we trying to match?  see i386_align_code in:
http://sourceware.org/cgi-bin/cvsweb.cgi/src/gas/config/tc-i386.c?annotate=1.460&cvsroot=src

The smart noppery is turned on via the special casing of 0x90
at line 438 in md_do_align in:
http://sourceware.org/cgi-bin/cvsweb.cgi/src/gas/config/tc-i386.h?annotate=1.1&cvsroot=src

tests/asmtest.S

@@ -9,7 +9,8 @@
    .int 1, 2, 3
    .align 8
    .byte 1
-   .align 16, 0x90
+/* .align 16, 0x90 gas is too clever for us with 0x90 fill */
+   .align 16, 0x91 /* 0x91 tests the non-clever behaviour */
    .skip 3
    .skip 15, 0x90
    .string "hello\0world"