Add GNU extensions to flang library

[hawkinsp]

A number of GNU extensions to the Fortran standard library are used in practice by various packages.

So far when building a corpus of opensource Fortran code I've noticed the lack of:

• etime (example user: LAPACK )
• rshift (example user: AVL )
• secnds (example user: AVL )
• getpid(example user: AIRSS )
• hostnm (example user: AIRSS )

[llvmbot]

@llvm/issue-subscribers-flang-runtime

[llvmbot]

@llvm/issue-subscribers-flang-ir

[mjklemm]

ETIME is tracked here: #84205.