[flang] Add default implementation for SYSTEM_CLOCK

Add an implementation for the runtime functions related to SYSTEM_CLOCK.
As with CPU_TIME, this is based on std::clock(), which should be
available everywhere, but it is highly recommended to add
platform-specific implementations for systems where std::clock() behaves
poorly (e.g. POSIX).

The documentation for std::clock() doesn't specify a maximum value and
in fact wrap around behaviour is non-conforming.  Therefore, this
implementation of SYSTEM_CLOCK is not guaranteed to wrap around either,
and after std::clock reaches its maximum value we will likely just
return failure rather than wrap around. If this happens often on your
system, please add a new platform-specific implementation.

We define COUNT_MAX as either the maximum value that can be stored in
a std::clock_t or in a 64-bit integer (whichever is smaller), and
COUNT_RATE as CLOCKS_PER_SEC. For POSIX systems, the value of
CLOCKS_PER_SEC is hardcoded to 10^6 and irrelevant for the values
returned by std::clock.

Differential Revision: https://reviews.llvm.org/D105969