gdb/doc: describe x87 registers

gdb/doc: describe x87 registers

While investigating this [1], I initially had no idea what register
"fioff" stood for, making it difficult to map it to something in the
Intel or AMD manuals.  Similarly, I can imaging someone familiar with
x87 to want to print the "x87 last instruction address", and have no
clue that GDB makes it available as register "fioff".  The names of the
x87 state fields don't seem to be standardized, they even change between
sections of the Intel manual (between the FSAVE, FXSAVE and XSAVE area
descriptions).

Add some details to the doc to help one map GDB register names to x87
state fields.

[1] https://inbox.sourceware.org/gdb-patches/20230908022722.430741-1-simon.marchi@efficios.com/T/#u

Change-Id: I0ea1eb648358e62da4aa87eea3515ee8a09f2762
Approved-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Pedro Alves <pedro@palves.net>