arc: Add GNU/Linux support for ARC

arc: Add GNU/Linux support for ARC

ARC Linux targets differences from baremetal:

- No support for hardware single instruction stepping.
- Different access rules to registers.
- Use of another instruction for breakpoints.

v2: Changes after Tom's remarks [1]
 arc-linux-tdep.c
  - Use true/false instead of TRUE/FALSE.
  - arc_linux_sw_breakpoint_from_kind (): Break long lines into two.
  - arc_linux_sw_breakpoint_from_kind (): Remove starting blank line.
  - Use explicit number evaluation, e.g: if (a & b) -> if ((a & b) != 0)
 arc-tdep.c
  - Use explicit number evaluation, e.g: if (a & b) -> if ((a & b) != 0)
 gdb/configure.tgt
  - arc*-*-linux*): Remove "build_gdbserver=yes".

v3: Changes after Simon's remarks [2]
  arc-linux-tdep.c
  - Use "return trap_size" instead of cryptic "return 2".
  - Removed unnecessary curly braces.
  - Removed "void" from "_initialize_arc_linux_tdep (void)".

v5: Changes after Simon's remarks [3]
- Remove unnecessary empty lines.
- Replace "breakpoint uses" with "breakpoints use" in a comment.
- "return condition;" i.s.o. "if (condition) return true; else return false;"

[1] Tom's remarks
https://sourceware.org/pipermail/gdb-patches/2020-April/167887.html

[2] Simon's remarks on v2
https://sourceware.org/pipermail/gdb-patches/2020-May/168513.html

[3] Simon's remarks on v4
https://sourceware.org/pipermail/gdb-patches/2020-August/170994.html

gdb/ChangeLog:

2020-08-25  Anton Kolesov  <anton.kolesov@synopsys.com>

	* configure.tgt: ARC support for GNU/Linux.
	* Makefile.in (ALL_TARGET_OBJS): Likewise.
	* arc-linux-tdep.c: New file.
	* arc-tdep.h (ARC_STATUS32_L_MASK, ARC_STATUS32_DE_MASK): Declare.
	* arc-tdep.c (arc_write_pc): Use it.