README: Add ARC architecture

ARC support was added back in 2013 but I missed updating here
Reported-by: NFrancois Bedard <fbedard@synopsys.com>
Signed-off-by: NVineet Gupta <vgupta@synopsys.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

README

@@ -24,7 +24,7 @@ ON WHAT HARDWARE DOES IT RUN?
   today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC and
   UltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, Cell,
   IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64, AXIS CRIS,
-  Xtensa, Tilera TILE, AVR32 and Renesas M32R architectures.
+  Xtensa, Tilera TILE, AVR32, ARC and Renesas M32R architectures.
 
   Linux is easily portable to most general-purpose 32- or 64-bit architectures
   as long as they have a paged memory management unit (PMMU) and a port of the