target/hppa: Drop attempted gdbstub support for hppa64

target/hppa: Drop attempted gdbstub support for hppa64

There is no support for hppa64 in gdb.  Any attempt to provide the
data for the larger hppa64 registers results in an error from gdb.
Mask CR_SAR writes to the width of the register: 5 or 6 bits.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>