Improve Morello feature detection

Improve Morello feature detection

Given HWCAP2_MORELLO changed for Linux Kernel 5.18, this breaks Morello GDB's
heuristic for detecting the Morello feature.

When possible, switch to detecting the Morello feature through the availability
of the NT_ARM_MORELLO register set (which means PTRACE_PEEKCAP and
PTRACE_POKECAP are also available).  For corefiles, switch to using the presence
of the Morello register set section.

For extended-remote mode, check for the two possible values of HWCAP2_MORELLO.