ARM/FDPIC: Add gdbserver support

ARM/FDPIC: Add gdbserver support

This patch extends the existing support for FDPIC on bfin/frv/sh to arm.
It is enabled when configuring with --host arm-none-uclinuxfdpiceabi,
using arm-none-uclinuxfdpiceabi-gcc.

We also change the way HAS_NOMMU is defined to make sure we use vfork
instead of fork (which is not available in the ARM FDPIC
configuration).

2021-04-14  Mickael Guene <mickael.guene@st.com>
	Christophe Lyon <christophe.lyon@st.com>

	* gdb/common/linux-ptrace.h: Define PTRACE for FDPIC for arm too.
	* gdb/gdbserver/configure.srv: Accept arm*-*-uclinuxfdpiceabi
	target.
	* gdb/gdbserver/linux-low.c (elf32_fdpic_loadseg): New.
	(elf32_fdpic_loadmap): New.
	(HAS_NOMMU): Fix definition.
	(linux_read_offsets): Add support for arm FDPIC.