[libc] Add missing dependency for test target (#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.
