Compiler-explorer: Add clang arm64 bit compiler support libraries

[obfuscated]

It will be useful to compare clang and GCC's code generators for arm 64bit code.

[nico]

FWIW you can already add a -target arg to the x86 clang to get arm64 output: https://godbolt.org/g/WJQWhe

[Ferdi265]

@nico: the -target hack doesn't work with C++ standard library includes though: https://godbolt.org/z/G7cb4P

[rengolin]

would be good to have at least these options for "arm clang":

• armv7a-linux-gnueabi (32-bits)
• armv8a-linux-gnueabi (32-bits)
• aarch64-linux-gnu (64 bits)

Thumb support (#1271) can be done, I suppose, by adding -mthumb and fiddling with march/mcpu.

This could be a wrapper for @nico's solution, but it would also have to know where the headers and libraries are, as per @Ferdi265's comment, which users don't.

[mattgodbolt]

See also #915

[apmorton]

This is fixed by #2262

https://godbolt.org/z/Yrff7G