[libc] fix -Wmissing-braces (#77345)

Fixes the following errors observed on the aarch64 fullbuild:


/home/libc-buildbot/libc-aarch64-ubuntu/libc-aarch64-ubuntu-fullbuild-dbg/llvm-project/libc/src/__support/HashTable/generic/bitmask_impl.inc:116:13:
    error: suggest braces around initialization of subobject
    [-Werror,-Wmissing-braces]
return {static_cast<bitmask_t>(mask_available().word ^
repeat_byte(0x80))};
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{ }
    In file included from

/home/libc-buildbot/libc-aarch64-ubuntu/libc-aarch64-ubuntu-fullbuild-dbg/llvm-project/libc/src/search/hdestroy.cpp:10:

/home/libc-buildbot/libc-aarch64-ubuntu/libc-aarch64-ubuntu-fullbuild-dbg/llvm-project/libc/src/__support/HashTable/table.h:336:41:
    error: suggest braces around initialization of subobject
    [-Werror,-Wmissing-braces]
      iterator end() const { return {0, 0, {0}, *this}; }
                                            ^
                                            {}

Link:
https://lab.llvm.org/buildbot/#/builders/223/builds/33868/steps/6/logs/stdio
Link: https://github.com/llvm/llvm-project/pull/74506