[Flang] Define PIC/PIE macros

[vzakhari]

gfortran defines __pic__/__PIC__/__pie__/__PIE__ macros according to -fpic/PIC/pie/PIE options, so I think flang should do the same.

Reference: https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fpic

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Slava Zakharin (vzakhari)

`gfortran` defines `__pic__/__PIC__/__pie__/__PIE__` macros according to `-fpic/PIC/pie/PIE` options, so I think `flang` should do the same.

Reference: https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fpic