llvm-readobj: fix printing number of relocations in Android packed format.

With '-elf-output-style=GNU -relocations', a header containing the number
of entries is printed before all the relocation entries in the section.
For Android packed format, we need to perform the unpacking first before
we can get the actual number of relocations in the section.

Patch by Rahul Chaudhry!

Differential Revision: https://reviews.llvm.org/D47800

llvm-svn: 334147