Fix build with latest GCC 9.0 tree

Fix build with latest GCC 9.0 tree

A recent patch [1] to fix a GCC PR [2] actually broke the GDB build.
To fix, remove the stack pointer clobber. GCC will ignore the clobber
marker, and will not save or restore the stack pointer.

I ran "make check-gdb" on x86_64 to ensure there are no regressions.

gdb/ChangeLog:

2018-12-17  Dimitar Dimitrov  <dimitar@dinux.eu>

	* nat/linux-ptrace.c (linux_ptrace_test_ret_to_nx): Remove sp clobbers.

[1] https://gcc.gnu.org/ml/gcc-patches/2018-12/msg00532.html
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52813

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>