[gdb/contrib] Add two words to common-misspellings.txt

While reviewing changes generated by spellcheck.sh for directory sim, I
noticed two more misspellings:
...
arrithemetic->arithmetic
electricaly->electrically
...

Add them to common-misspellings.txt, and fix them in directory sim.

Tested by rebuilding on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/contrib/common-misspellings.txt

@@ -22,6 +22,8 @@ sofar->so far
 doens't->doesn't
 behavour->behavior
 behaviour->behavior
+arrithemetic->arithmetic
+electricaly->electrically
 
 # Identity rules.
 

sim/ppc/INSTALL

@@ -32,7 +32,7 @@ following:
 
 	gcc			GCC version two includes suport
 				for long long (64bit integer)
-				arrithemetic which PSIM uses.  Hence
+				arithmetic which PSIM uses.  Hence
 				it is recommended that you build PSIM
 				using GCC.
 				

sim/ppc/hw_eeprom.c

@@ -29,7 +29,7 @@
 /* DEVICE
 
 
-   eeprom - JEDEC? compatible electricaly erasable programmable device
+   eeprom - JEDEC? compatible electrically erasable programmable device
 
 
    DESCRIPTION

sim/ppc/idecode_expression.h

@@ -40,7 +40,7 @@
    resulting dead code.  That dead code being the calculations that,
    as it turned out were not in the end needed.
 
-   64bit arrithemetic is used firstly because it allows the use of
+   64bit arithmetic is used firstly because it allows the use of
    gcc's efficient long long operators (typically efficiently output
    inline) and secondly because the resultant answer will contain in
    the low 32bits the answer while in the high 32bits is either carry
@@ -48,7 +48,7 @@
 
 /* 64bit target expressions:
 
-   Unfortunately 128bit arrithemetic isn't that common.  Consequently
+   Unfortunately 128bit arithmetic isn't that common.  Consequently
    the 32/64 bit trick can not be used.  Instead all calculations are
    required to retain carry/overflow information in separate
    variables.  Even with this restriction it is still possible for the