[llvm-ar][Object] Fix detection of need for 64-bit archive symbol tables

The code to detect the requirement for 64-bit offsets in the archive
symbol table was not correctly accounting for the archive file signature
and the size of all the contents of the symbol table itself, e.g. the
symbol table's header and string table. Also was not considering the
variation in symbol table formats. This could result in the creation of
large archives with a corrupt symbol table.

Change the testing environment variable SYM64_THRESHOLD to be an
absolute value rather than a power of 2 in order to enable precise
testing of this detection code.

Differential Revision: https://reviews.llvm.org/D89891