gdbstub: Introduce gdb_get_float64() to get 64-bit float registers

gdbstub: Introduce gdb_get_float64() to get 64-bit float registers

When converted to use GByteArray in commits 462474d760c and
a010bdbe719, the call to stfq_p() was removed. This call
serialize a float.
Since we now use a GByteArray, we can not use stfq_p() directly.
Introduce the gdb_get_float64() helper to load a float64 register.

Fixes: 462474d760c ("target/m68k: use gdb_get_reg helpers")
Fixes: a010bdbe719 ("extend GByteArray to read register helpers")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200414163853.12164-3-philmd@redhat.com>
Message-Id: <20200430190122.4592-3-alex.bennee@linaro.org>