sys/syz-extract: fix linux/arm

At some point extract for linux/arm started to fail with:

extracting from key.txt
	failed to run compiler: exit status 1
/tmp/ccT6o7iZ.s: Assembler messages:
/tmp/ccT6o7iZ.s:4: Error: unknown pseudo-op: `.syntax'

This is caused by arch/arm/include/asm/unified.h:

__asm__(".syntax unified");

Ignore all __asm__ directives as we are cross compiling.

sys/syz-extract/fetch.go

@@ -133,6 +133,8 @@ func compile(cc string, args []string, data *CompileData) (bin string, out []byt
 }
 
 var srcTemplate = template.Must(template.New("").Parse(`
+#define __asm__(...)
+
 {{range $incl := $.Includes}}
 #include <{{$incl}}>
 {{end}}