[libc] fix strtointmax tests

The strtointmax tests weren't running because they depend on the same
shared logic as strtol tests and the target for that was being defined
after the inttypes targets, causing them to be skipped. This patch moves
the inttypes test directory later in the order so that the shared tests
are defined before inttypes tests try to use them.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D142515