[libcxx] [test] Fix max_size.pass.cpp for PowerPC targets

This patch fixes the max_size.pass.cpp test for PowerPC targets, depending on
endianness.

We will exhibit the full_size() behaviour for little endian
(where __endian_factor = 2 ), and the half_size() behaviour for
big endian (where __endian_factor = 1).

Differential Revision: https://reviews.llvm.org/D131682