Before this commit, cc-wrapper/default.nix was using
`isGccArchSupported` to validate `-mtune=` values.  This has two
problems:

- On x86, `-mtune=` can take the same values as `-march`, plus two
  additional values `generic` and `intel` which are not valid for
  `-march`.

- On ARM, `-mtune=` does not take the same values as `-march=`;
  instead it takes the same values as `-mcpu`.

This commit fixes these two problems by adding a new
`isGccTuneSupported` function.  For `isx86` this returns `true` for
the two special values and otherwise defers to `isGccArchSupported`.

This commit also adds support for `-mtune=` on Aarch64.

Unfortunately on Aarch64, Clang does not accept as wide a variety of
`-mtune=` values as Gcc does.  In particular, Clang does not tune
for big.LITTLE mixed-model chips like the very popular RK3399, which
is targeted using `-march=cortex-a72.cortex-a53` in gcc.

To address this problem, this commit also adds a function
`findBestTuneApproximation` which can be used to map
clang-unsupported tunings like `cortex-a72.cortex-a53` to
less-precise tunings like `cortex-a53`.

The work which led to this commit arose because we now have
packages, like `crosvm`, which use *both* `clang` *and* `gcc`.
Previously I had been using `overrideAttrs` to set
`NIX_CFLAGS_COMPILE` on a package-by-package basis based on which
compiler that package used.  Since we now have packages which use
*both* compilers, this strategy no longer works.

I briefly considered splitting `NIX_CFLAGS_COMPILE` into
`NIX_CFLAGS_COMPILE_GCC` and `NIX_CFLAGS_COMPILE_CLANG`, but since
`NIX_CFLAGS_COMPILE` is sort of a hack to begin with I figured that
adding the logic to `cc-wrapper` would be preferable.