Support assembly directives

[dylanmckay]

Add support for all AVR assembly directives.

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List of directives (source):

Directive	Priority	Description	Status
BYTE	High	Reserve byte to a variable
CSEG	High	Code Segment
CSEGSIZE		Program memory size
DB	High	Define constant byte(s)
DEF		Define a symbolic name on a register
DEVICE		Define which device to assemble for
DSEG	High	Data Segment
DW	High	Define Constant word(s)
ENDM, ENDMACRO		End macro
EQU	High	Set a symbol equal to an expression
ESEG	Medium	EEPROM Segment
EXIT		Exit from file
INCLUDE	Medium	Read source from another file
LIST		Turn listfile generation on
LISTMAC		Turn Macro expansion in list file on
MACRO		Begin macro
NOLIST		Turn listfile generation off
ORG	High	Set program origin
SET	High	Set a symbol to an expression

[dylanmckay]

I believe that most (if not all) are already handled by LLVM. I believe the hi8, lo8 macros are already handled by AVR-LLVM (see AVRMCExpr.h). Support for the hh8 macro may need to be added.

[dylanmckay]

In PR #105, it was shown that we are only missing two directives: .list and .nolist. They are also general to all GAS targets.

Closing, as these are not important, nor do they belong in this issue tracker.