[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

(cherry picked from commit e2e9e2c)

Author: amy-kwan

libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp

@@ -74,7 +74,13 @@ bool test() {
 #   else
   full_size();
 #   endif
-# elif defined(__powerpc__) || defined(__powerpc64__) || defined(__sparc64__)
+# elif defined(__powerpc__) || defined(__powerpc64__)
+#   ifdef __BIG_ENDIAN__
+  half_size();
+#   else
+  full_size();
+#   endif
+# elif defined(__sparc64__)
   half_size();
 # elif defined(_WIN32)
   full_size();