[libc][test] Fix death test timeouts and dead-code elimination in math tests. (#227966)

- In `ExecuteFunctionUnix.cpp`, call `prctl(PR_SET_DUMPABLE, 0)` in
child processes to prevent external core dump handlers (such as apport)
from intercepting expected crashes during death tests, eliminating 10s
poll timeouts under parallel lit runs.
- Close `pipe_fds[0]` properly in `invoke_in_subprocess` to avoid
leaking file descriptors.
- In math smoke test templates (`AddTest.h`, `SubTest.h`, `MulTest.h`,
`DivTest.h`), assign test function results to `[[maybe_unused]] volatile
OutType res` to prevent GCC from dead-code eliminating floating point
operations in `test_inexact_results` when FMA optimization is disabled.

Assisted-by: Gemini
GitOrigin-RevId: 0015ba584055f39bc729424ae77e3f5543169543