arm: add __aeabi_unwind_cpp_pr1() function to avoid linker complaints

With the ELDK 5.1 (beta) "armv7a" toolchain I'm get the following
build failure:

  $ ./MAKEALL mx51evk
  ...
  /opt/eldk-5.1/armv7a/sysroots/arm-linux-gnueabi/usr/lib/arm-linux-
  gnueabi/4.6.1/libgcc.a(bpabi.o):(.ARM.exidx+0x0): undefined
  reference to `__aeabi_unwind_cpp_pr1'
  make: *** [u-boot] Error 1

This patch fixes the issue similar to commit
d442b6e7ad6a86e2fd0e6297291fe8872ff26fc6 but I don't know if it's
general enough or if it does harm when using other toolchains.

Signed-off-by: Wolfgang Grandegger <wg@denx.de>