[ld, testsuite] XFAIL "pr20995-2", "Build pr20995-2.so" on arm*-*-eabi*

ld/
	* testsuite/ld-elf/elf.exp ("pr20995-2", "Build pr20995-2.so"): XFAIL on
	arm*-*-eabi*.

Some technical discussions about the reason of this XFAIL are available at

  https://sourceware.org/ml/binutils/2017-01/msg00441.html