Base option for executable space-optimizations

[mitchgrout]

A common pattern within embedded firmware projects is to use -ffunction-sections and -fdata-sections to hoist each symbol into its own dedicated linker section, then use -Wl,--gc-sections to inform the linker to discard any section without a reference[1]. This allows for the use of a particularly large, feature-rich library, without having to pay the space cost of keeping every function. Obviously, this only works if each library is compiled from source, as an already-compiled library cannot be stripped (AFAIK).

The annoying part of this is, the -Wl,--gc-sections should only be applied to final executable() objects. If applied to a library(), it's prone to strip out otherwise necessary symbols. Writing it in meson currently, you end up with the two arguments being quite far apart, e.g.

project(...)
add_global_arguments('-ffunction-sections', '-fdata-sections', language: ['c', 'cpp'])

# set up dependencies...
# set up libraries...
# quite a few lines later...
exe = executable(
  'my-project',
  sources...,
  dependencies: dependencies...,
  link_args: ['-Wl,--gc-sections'],
)

I'd also argue that this config isn't really a project-specific thing; it's rather ubiquitous. As such, making it into a dedicated option feels like the meson way. This option would:

1. Apply -ffunction-sections and -fdata-sections as a global compiler argument, if supported by the compiler, and
2. Apply -Wl,--gc-sections as a per-target argument, if the target is an executable()

I'm not aware if this optimization exists for other languages like Rust, but with b_sanitize existing, a C/C++ specific option does not seem totally unreasonable.

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[1] https://interrupt.memfault.com/blog/best-and-worst-gcc-clang-compiler-flags#-ffunction-sections--fdata-sections----gc-sections