[libc] fix -Wcast-function-type via union rather than reinterpret_cast (#76875)

The GCC build is producing the following diagnostic:

llvm-project/libc/src/signal/linux/signal_utils.h: In member function
    ‘__llvm_libc_18_0_0_git::KernelSigaction&
__llvm_libc_18_0_0_git::KernelSigaction::operator=(const sigaction&)’:
    llvm-project/libc/src/signal/linux/signal_utils.h:38:20: warning:
cast between incompatible function types from ‘void (*)(int, siginfo_t*,
    void*)’ to ‘void (*)(int)’ [-Wcast-function-type]
38 | sa_handler = reinterpret_cast<HandlerType *>(sa.sa_sigaction);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
llvm-project/libc/src/signal/linux/signal_utils.h: In member function
‘__llvm_libc_18_0_0_git::KernelSigaction::operator sigaction() const’:
    llvm-project/libc/src/signal/linux/signal_utils.h:51:25: warning:
cast between incompatible function types from ‘void (*)(int)’ to ‘void
    (*)(int, siginfo_t*, void*)’ [-Wcast-function-type]
51 | s...