gdb/x86: Handle kernels using compact xsave format

gdb/x86: Handle kernels using compact xsave format

For GNU/Linux on x86-64, if the target is using the xsave format for
passing the floating-point information from the inferior then there
currently exists a bug relating to the x87 control registers, and the
mxcsr register.

The xsave format allows different floating-point features to be lazily
enabled, a bit in the xsave format tells GDB which floating-point
features have been enabled, and which have not.

Currently in GDB, when reading the floating point state, we check the
xsave bit flags, if the feature is enabled then we read the feature
from the xsave buffer, and if the feature is not enabled, then we
supply the default value from within GDB.

Within GDB, when writing the floating point state, we first fetch the
xsave state from the target and then, for any feature that is not yet
enabled, we write the default values into the xsave buffer.  Next we
compare the regcache value with the value in the xsave buffer, and, if
the value has changed we update the value in the xsave buffer, and
mark the feature enabled in the xsave bit flags.

The problem then, is that the x87 control registers were not following
this pattern.  We assumed that these registers were always written out
by the kernel, and we always wrote them out to the xsave buffer (but
didn't enabled the feature).  The result of this is that if the kernel
had not yet enabled the x87 feature then within GDB we would see
random values for the x87 floating point control registers, and if the
user tried to modify one of these register, that modification would be
lost.

Finally, the mxcsr register was also broken in the same way as the x87
control registers.  The added complexity with this case is that the
mxcsr register is part of both the avx and sse floating point feature
set.  When reading or writing this register we need to check that at
least one of these features is enabled.

This bug was present in native GDB, and within gdbserver.  Both are
fixed with this commit.

gdb/ChangeLog:

	* common/x86-xstate.h (I387_FCTRL_INIT_VAL): New constant.
	(I387_MXCSR_INIT_VAL): New constant.
	* amd64-tdep.c (amd64_supply_xsave): Only read state from xsave
	buffer if it was supplied by the inferior.
	* i387-tdep.c (i387_supply_fsave): Use I387_MXCSR_INIT_VAL.
	(i387_xsave_get_clear_bv): New function.
	(i387_supply_xsave): Only read x87 control registers from the
	xsave buffer if the feature is enabled, and the state will have
	been written, otherwise, provide a suitable default.
	(i387_collect_xsave): Pre-clear all registers in xsave buffer,
	including x87 control registers.  Update control registers if they
	have changed from the default value, and mark features as enabled
	as required.
	* i387-tdep.h (i387_xsave_get_clear_bv): Declare.

gdb/gdbserver/ChangeLog:

	* i387-fp.c (i387_cache_to_xsave): Only write x87 control
	registers to the cache if their values have changed.
	(i387_xsave_to_cache): Provide default values for x87 control
	registers when these features are available, but disabled.
	* regcache.c (supply_register_by_name_zeroed): New function.
	* regcache.h (supply_register_by_name_zeroed): Declare new
	function.

gdb/testsuite/ChangeLog:

	* gdb.arch/amd64-init-x87-values.S: New file.
	* gdb.arch/amd64-init-x87-values.exp: New file.