x86: Use correct .reg-xstate section size

x86: Use correct .reg-xstate section size

When reading the XSAVE extended state from an i386 or AMD64 core file,
the respective regset iterator requests a minimum section size of
zero.  Since the respective regset supply function does not check the
size either, this may lead to accessing data out of range if the
section is too short.

In write mode, the iterator always uses the maximum supported size for
the XSAVE extended state.

This is now changed such that the iterator always requests the
expected size of this section based on xcr0, both for reading and
writing.

gdb/ChangeLog:

	* amd64-linux-tdep.c (amd64_linux_iterate_over_regset_sections):
	For ".reg-xstate", explicitly specify the requested section size
	via X86_XSTATE_SIZE instead of just 0 on input and
	X86_XSTATE_MAX_SIZE on output.
	* i386-linux-tdep.c (i386_linux_iterate_over_regset_sections):
	Likewise.