better libc detection (#1996)

* better libc detection

This introduces a new command `zig libc` which prints
the various paths of libc files. It outputs them to stdout
in a simple text file format that it is capable of parsing.
You can use `zig libc libc.txt` to validate a file.

These arguments are gone:
--libc-lib-dir [path]        directory where libc crt1.o resides
--libc-static-lib-dir [path] directory where libc crtbegin.o resides
--msvc-lib-dir [path]        (windows) directory where vcruntime.lib resides
--kernel32-lib-dir [path]    (windows) directory where kernel32.lib resides

Instead we have this argument:
--libc [file]                Provide a file which specifies libc paths

This is used to pass a libc text file (which can be generated with
`zig libc`). So it is easier to manage multiple cross compilation
environments.

`--cache on` now works when linking against libc.

`ZigTarget` now has a bool field `is_native`

Better error messaging when you try to link against libc or use
`@cImport` but the various paths cannot be found. It should also be
faster.

* save native_libc.txt in zig-cache

This avoids having to detect libc at runtime on every invocation.

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