[llvm-ar] Fix support for archives with members larger than 4GB

llvm-ar outputs a strange error message when handling archives with
members larger than 4GB due to not checking file size when passing the
value as an unsigned 32 bit integer. This overflow issue caused
malformed archives to be created.:

https://bugs.llvm.org/show_bug.cgi?id=38058

This change allows for members above 4GB and will error in a case that
is over the formats size limit, a 10 digit decimal integer.

Differential Revision: https://reviews.llvm.org/D65093

llvm-svn: 366813