meson.build: fix wrong .symver detection

meson.build: fix wrong .symver detection

As pointed here [1] __has_attribute() is broken for many attributes and
if it doesn't support the specific attribute it returns true, so we
can't really rely on that for this check. This lead to Buildroot
libfuse3 build failure [2] where that shows up with:
```
error: symver is only supported on ELF platforms
```
Indeed Microblaze doesn't support ELF since it doesn't include elfos.h,
but __has_attribute(symver) returns true.

So let's substitute the #ifdef __has_attribute() with a stronger test on
a function foo() with __attribute__((symver ("test@TEST"))).

[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101766
[2]: http://autobuild.buildroot.net/results/d6c/d6cfaf2aafaeda3c12d127f6a2d2e175b25e654f/build-end.log

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>