[libc] Run all unit tests, irrespective of whether they belong to a test suite.

Previously, only those unit tests which belonged to a suite were run as
part of libc-unit-tests. It meant that unit tests not part of any suite
were not being tested. This change makes all unit tests run as part of
libc-unit-tests. The convenience function to add a libc unit test suite
has been removed and add_custom_target is used in its place. One of the
bit-rotting test has along been fixed. Math exhaustive and differential
tests are skipped under full build.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148784