[Multilib] Custom flags YAML parsing (#122903)

This patch is the first step to extend the current multilib system to
support the selection of library variants which do not correspond to
existing command-line options.

Proposal can be found in
https://discourse.llvm.org/t/rfc-multilib-custom-flags/81058

The multilib mechanism supports libraries that target code generation or
language options such as `--target`, `-mcpu`, `-mfpu`,
`-mbranch-protection`. However, some library variants are particular to
features that do not correspond to any command-line options. Examples
include variants for multithreading and semihosting.

This work introduces a way to instruct the multilib system to consider
these features in library selection. This particular patch comprises a
new section in `multilib.yaml` to declare flags for which no option
exists. Henceforth this sort of flag will be called `custom flag` for
clarity.

The `multilib.yaml` file will have a new section called Flags which
contains the declarations of the target’s custom flags:

```yaml
Flags:
- Name: multithreaded
  Values:
  - Name: no-multithreaded
    MacroDefines: [__SINGLE_THREAD__]
  - Name: multithreaded
  Default: no-multithreaded

- Name: io
  Values:
    - Name: io-none
    - Name: io-semihosting
      MacroDefines: [SEMIHOSTING]
    - Name: io-linux-syscalls
      MacroDefines: [LINUX_SYSCALLS, HOSTED=1]
   Default: io-none
```
- Name: the name to categorize a flag.
- Values: a list of possible values.
- Default: it specifies which value this flag should take if not
specified in the command-line invocation. It must be one value from the
Values field.

Each flag Value follows this description:
- Name (required): the name of the custom flag value (string). This is
the string to be used in `-fmultilib-flag=<string>`.
- MacroDefines (optional): a list of strings to be used as macro
definitions. Each string
  is fed into the driver as ``-D<string>``.

A Default value is useful to save users from specifying custom flags
that have a most commonly used value.

The namespace of flag values is common across all flags. This means that
flag values must be unique.

0 files changed, 0 insertions, 0 deletions