[libc] Add missing dependency for test target (llvm#138423)

It seems we were missing a dependency when adding a new test target,
e.g., test libc.test.src.math.sqrt_test.__unit__ would create a
custom target in the form of:

```
  add_custom_target(
    libc.test.src.__support.FPUtil.dyadic_float_test.__unit__
    COMMAND  ${LIBC_UNITTEST_ENV} ${CMAKE_CROSSCOMPILING_EMULATOR} libc.test.src.__support.FPUtil.dyadic_float_test.__unit__.__build__
    COMMENT Running unit test libc.test.src.__support.FPUtil.dyadic_float_test.__unit__ )
``` 

but it wouldn't set that it depends on
libc.test.src.__support.FPUtil.dyadic_float_test.__unit__.__build__
being built.

For some reason, it would break the rv32 buildbot, as it would try to
run a test but the __build__ is nowhere to be found, since it wasn't
built in the first place.

Author: mikhailramalho

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -321,6 +321,7 @@ function(create_libc_unittest fq_target_name)
       ${fq_target_name}
       COMMAND ${LIBC_UNITTEST_ENV} ${CMAKE_CROSSCOMPILING_EMULATOR} ${fq_build_target_name}
       COMMENT "Running unit test ${fq_target_name}"
+      DEPENDS ${fq_build_target_name}
     )
   endif()