compilers: support GCC incremental LTO

GCC 15 supports incremental LTO (caching). It's the same feature as
Clang's ThinLTO caching.

Unfortunately, for the Clang ThinLTO cache support, we chose a pretty
vendor-specific option name, and I don't want to worry about deprecating
that (-Db_thinlto_cache and -Db_thinlto_cache_dir) at this time, so...

Wire -Db_thinlto_cache and -Db_thinlto_cache_dir for GCC in the obvious
and natural way.

Other notes:

* GCC, unlike Clang, requires the cache dir to exist. If the
  user passes an invalid directory, that's on them, but for our default
  in meson-private, we should os.mkdir() it, so we do that now too.

* I did wonder about adding -flto-partition=cache, but when I asked about
  this last year [0], the response was that it has too many caveats. Though
  Honza did recently [1] suggest it has some utility, so I don't know. I'll
  ask about it.

[0] https://inbox.sourceware.org/gcc-patches/vjeanwm7b2jnedzmbwww4cy6jf4prty6ypxvz7x5d463xw2mys@bxzxuvgfaplf/
[1] https://inbox.sourceware.org/gcc-patches/aLnAGpln6SXbjV7e@kam.mff.cuni.cz/

Closes: https://github.com/mesonbuild/meson/issues/14428

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