coregrind/m_debuglog.c: fix incorrect inline assembly for amd64-linux.

coregrind/m_debuglog.c: fix incorrect inline assembly for amd64-linux.

This adds rcx and r11 to the trash lists for inline assembly in
local_sys_write_stderr and local_sys_getpid, since those registers are not
preserved across the embedded syscall that takes us into the kernel.  I
believe this to be the root cause of the apparent code generation bug
addressed in 53faacfda4ad6a27bc0e8ab859c1cc5388a61a5e (16 Nov 2017).  This
commit also undoes that commit, because I think it is no longer necessary.