on armv7h, /usr/include/bits/syscall.h defines SYS_mmap2 to __NR_mmap2:

$ grep mmap2 /usr/include/bits/syscall.h
#define SYS_mmap2 __NR_mmap2
#define SYS_mmap2 __NR_mmap2
$ pacman -Qo /usr/include/bits/syscall.h
/usr/include/bits/syscall.h is owned by glibc 2.26-10

but __NR_mmap2 is only defined in /usr/include/asm/unistd_32.h, which is not included on armv7h, as per /usr/include/asm/unistd.h:

# ifdef __i386__
#  include <asm/unistd_32.h>
# elif defined(__ILP32__)
#  include <asm/unistd_x32.h>
# else
#  include <asm/unistd_64.h>
# endif

both of which are part of linux-libre-api-headers:

$ pacman -Qo /usr/include/asm/unistd.h 
/usr/include/asm/unistd.h is owned by linux-libre-api-headers 4.12.7_gnu-1
$ pacman -Qo /usr/include/asm/unistd_32.h 
/usr/include/asm/unistd_32.h is owned by linux-libre-api-headers 4.12.7_gnu-1

I'm not sure what is going on there, but I think our linux-libre-api-headers appears to mismatch syscall numbers on arm? This currently causes the iceweasel build on arm to fail.