doc: gcc-plugins: update gcc-plugins.rst

This document was written a long time ago. Update it.

[1] Drop the version information

The range of the supported GCC versions are always changing. The
current minimal GCC version is 4.9, and commit 1e860048
("gcc-plugins: simplify GCC plugin-dev capability test") removed the
old code accordingly.

We do not need to mention specific version ranges like "all gcc versions
from 4.5 to 6.0" since we forget to update the documentation when we
raise the minimal compiler version.

[2] Drop the C compiler statements

Since commit 77342a02 ("gcc-plugins: drop support for GCC <= 4.7")
the GCC plugin infrastructure only supports g++.

[3] Drop supported architectures

As of v5.11-rc4, the infrastructure supports more architectures;
arm, arm64, mips, powerpc, riscv, s390, um, and x86. (just grep
"select HAVE_GCC_PLUGINS") Again, we miss to update this document when a
new architecture is supported. Let's just say "only some architectures".

[4] Update the apt-get example

We are now discussing to bump the minimal version to GCC 5. The GCC 4.9
support will be removed sooner or later. Change the package example to
gcc-10-plugin-dev while we are here.

[5] Update the build target

Since commit ce2fd53a ("kbuild: descend into scripts/gcc-plugins/
via scripts/Makefile"), "make gcc-plugins" is not supported.
"make scripts" builds all the enabled plugins, including some other
tools.

[6] Update the steps for adding a new plugin

At first, all CONFIG options for GCC plugins were located in arch/Kconfig.
After commit 45332b1b ("gcc-plugins: split out Kconfig entries to
scripts/gcc-plugins/Kconfig"), scripts/gcc-plugins/Kconfig became the
central place to collect plugin CONFIG options. In my understanding,
this requirement no longer exists because commit 9f671e58 ("security:
Create "kernel hardening" config area") moved some of plugin CONFIG
options to another file. Find an appropriate place to add the new CONFIG.

The sub-directory support was never used by anyone, and removed by
commit c17d6179

 ("gcc-plugins: remove unused GCC_PLUGIN_SUBDIR").

Remove the useless $(src)/ prefix.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>