docs/core-api: expand Fedora instructions for GCC plugins

In Fedora 36, cross-compiling an allmodconfig configuration
for other architectures on x86 fails with this problem:

In file included from ../scripts/gcc-plugins/gcc-common.h:95,
                 from ../scripts/gcc-plugins/latent_entropy_plugin.c:78:
/usr/lib/gcc/aarch64-linux-gnu/12/plugin/include/builtins.h:23:10: fatal
error: mpc.h: No such file or directory
   23 | #include <mpc.h>
      |          ^~~~~~~
compilation terminated.

In that distro, that header file is available in the separate
libmpc-devel package.

Although future versions of Fedora might correctly mark
that dependency, mention this additional package.

To help detect such problems ahead of time, describe the
    gcc -print-file-name=plugin
command that is used by scripts/gcc-plugins/Kconfig to detect
plugins [1].

[1] https://lore.kernel.org/lkml/CAHk-=wjjiYjCp61gdAMpDOsUBU-A2hFFKJoVx5VAC7yV4K6WYg@xxxxxxxxxxxxxx/

Fixes: 43e96ef8

 ("docs/core-api: Add Fedora instructions for GCC plugins");
Signed-off-by: Robert Elliott <elliott@hpe.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220827193836.2582079-1-elliott@hpe.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>