get_regset: define iff PTRACE_GETREGSET is used

get_regset: define iff PTRACE_GETREGSET is used

* syscall.c [AARCH64, X86_64, X32] (ARCH_REGS_FOR_GETREGSET,
ARCH_IOVEC_FOR_GETREGSET): New macros.
(get_regset): Define iff ARCH_REGS_FOR_GETREGSET is defined; change
return type to long, return ptrace() return code instead of assigning it
to get_regs_error; do not list individual architectures, implement
constant and variable iovec cases depending on ARCH_IOVEC_FOR_GETREGSET.
(get_regs): Assign get_regset() return code to get_regs_error.