Add constexpr specifier to read-only flag

constexpr is availble since C++11 and clang can take advantage
of it for a compile-time optimization.

If C++ version older than C++11 is used, we may need to emit
the constexpr specifier conditionally, e.g. using cpp_std specified
in .bp or wrapping it with a C macro.

Bug: 395633959
Test: atest aconfig.test
Change-Id: I826f07ade87847a8b3e188f300aff6907fcd76dd