Fix build failure and make mplayer work properly on NetBSD/arm:
- workaround name conflict with system header <arm/bswap.h> in libavutils
  by subst.mk replacement
- explicitly disable armv5te, armv6, and armv6t2 options that are enabled
  by autodetect code in configure but cause illegal instruction errors
  on StrongARM and XScale machines

Tested on NetBSD/hpcarm (WS011SH) and NetBSD/shark (Funai's DNARD) 5.99.38.

(tsutsui)