options: restore special behavior of CFLAGS vs. c_args

For compatibility with Autotools, CFLAGS is added to the linker command
line if the compiler acts as a linker driver.  However, this behavior
was lost in commit d37d649b0 ("Make all Meson level options overridable
per subproject.", 2025-02-13).

The issue is that (for example) c_link_args is stored in env.options, and
from that point on it is treated as a machine-file option.  This includes
not being able to override it in compilers.get_global_options:

- initialize_from_top_level_project_call places it in pending_options

- add_lang_args passes the right value to add_compiler_option

- add_compiler_option calls add_system_option_internal

- add_system_option_internal fishes the value out of pending_options
  and ignores what get_global_options provided.

Instead, store the putative values of the compiler options coming from
the environment in a separate dictionary, that is only accessed by
get_global_options. This way it never appears in pending_options, and
also there is no internal *_env_args variable anymore.

Fixes: #14533
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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