gdb/doc: rename "x86 Architecture-specific Issues" section to "x86"

gdb/doc: rename "x86 Architecture-specific Issues" section to "x86"

I'm looking to add some x86-specific information to the doc, but I find
the naming of this section odd.  It doesn't really talk about issues, it
just gives generally useful information.  Also, the sections about other
architectures don't mention "issues", just the architecture name.

Also, at least in the HTML version of the doc, the name is inconsistent
between the main table of content, where it appears as "x86
Architecture-specific Issues", and the sub-table of contents of the
"Architectures" section, where it appears as "i386".

Rename the section to just "x86".

Change-Id: I0a119ff1ab5e7b83c9afa3c3977eb085e88f52ca
Approved-By: Eli Zaretskii <eliz@gnu.org>