[clang] [flang] [flang] Expose -m64 option (PR #132409)

[Daniel Chen via cfe-commits]

DanielCChen wrote:

@JDPailleux @kiranchandramohan 
We are planning to add support for flang to compile 32-bit application as well as building 32-bit flang-rt on AIX. The first thing would be to enable `-m32` for flang in the driver. I saw Kiran's comment. Would it be possible to make `-m32` available in the driver but disable it in front_end if desired?   

https://github.com/llvm/llvm-project/pull/132409