Need a method like cc.compiles for assembler

[dcbaker]

in pixman we want to do some assembler tests, and the input is raw assembly with c pre-processor macros. This can be made to work by writing the test to a .S file and then passing it in, but cannot be made to work (currently) using a string passed to compiler.compiles.

One way to go about doing this would be to add a assembly flag to the compiler object, but I'm not sure if that's the right way to solve it, since that relies on specific behavior of GCC/Clang, and notably doesn't work with MSVC. A better way might be do do something similar for assemblers to what I'm doing in #5148, splitting the assembler into a separate representation and allowing the user to ask for the assembler like another compiler, doing something like:

as = meson.get_compiler('assembler')
works = as.compiles(
    '''my . made:up
      assembly . code
    """)

related: #4366