[flang-rt] Set _POSIX_C_SOURCE on Darwin

[luporl]

Clang on Darwin enables non-POSIX extensions by default.
This causes some macros to leak, such as HUGE from <math.h>, which
causes some conflicts with Flang symbols (but not with Flang-RT, for
now).

It also causes some Flang-RT extensions to be disabled, such as FDATE,
that checks for _POSIX_C_SOURCE. Setting _POSIX_C_SOURCE avoids these
issues. This is already being done in Flang, but it was not ported to
Flang-RT.

This also fixes check-flang-rt on Darwin, as NoArgv.FdateNotSupported
is broken since the flang runtime was moved to flang-rt.

Fixes #82036

[DavidTruby]

LGTM, thanks!

[llvm-ci]

LLVM Buildbot has detected a new failure on builder ppc64le-flang-rhel-clang running on ppc64le-flang-rhel-test while building flang-rt at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/40546

Here is the relevant piece of the build log for the reference

Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
...
PASS: Flang :: Transforms/dlti-dependency.fir (3836 of 3846)
PASS: Flang :: Driver/use-module.f90 (3837 of 3846)
PASS: Flang :: Transforms/DoConcurrent/basic_host.f90 (3838 of 3846)
PASS: Flang :: Transforms/debug-line-table.fir (3839 of 3846)
PASS: Flang :: Transforms/debug-dwarf-version.fir (3840 of 3846)
PASS: Flang :: Semantics/numeric_storage_size.f90 (3841 of 3846)
PASS: Flang :: Driver/fopenmp.f90 (3842 of 3846)
PASS: Flang :: Transforms/tbaa-local-alloc-threshold.fir (3843 of 3846)
PASS: Flang :: Transforms/stack-arrays.f90 (3844 of 3846)
PASS: Flang :: Driver/omp-driver-offload.f90 (3845 of 3846)
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1819.718996