Link to compiler explorer

[awvwgk]

We could link in the compiler section for each Fortran compiler available on compiler explorer (https://godbolt.org/) to the respective view to allow people to quickly give the compiler a try without having to install it first.

Currently available on compiler explorer are

• GCC GFortran (x86_64, aarch64, arm, mips, mipsel, mips64, mips64el, power, power64, power64le, s390x)
• LLVM flang (x86_64)
• Intel ifort / ifx (x86_64)

[certik]

We should also get LFortran into godbolt (lfortran/lfortran#623).

[ivan-pi]

There is an open issue at GodBolt to support fpm: compiler-explorer/compiler-explorer#3919