[flang] Disable libc++ assertions in the runtime library

Similar to D143168. Solve compiling error caused by D143612.

Error info:
```
flang-new -flang-experimental-exec hello.f90
/usr/bin/ld: /usr/bin/ld: DWARF error: invalid or unhandled FORM value: 0x25
llvm-project/build/lib/libFortranRuntime.a(unit.cpp.o): in function `std::__1::optional<unsigned long>::operator*[abi:v170000]() &':
unit.cpp:(.text._ZNRSt3__18optionalImEdeB7v170000Ev[_ZNRSt3__18optionalImEdeB7v170000Ev]+0x4f): undefined reference to `std::__1::__libcpp_verbose_abort(char const*, ...)'
/usr/bin/ld: llvm-project/build/lib/libFortranRuntime.a(unit.cpp.o): in function `void std::__1::__optional_storage_base<long, false>::__construct[abi:v170000]<unsigned long>(unsigned long&&)':
unit.cpp:(.text._ZNSt3__123__optional_storage_baseIlLb0EE11__constructB7v170000IJmEEEvDpOT_[_ZNSt3__123__optional_storage_baseIlLb0EE11__constructB7v170000IJmEEEvDpOT_]+0x55): undefined reference to `std::__1::__libcpp_verbose_abort(char const*, ...)'
```

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D143890