Gtirb-pprinter

* Fix bugs in printing shift instructions in AT&T syntax.
* Add `--use-gcc` option overriding `gcc` executable when binary printing ELF files.
* Fix printing symbols with a displacement of zero in ARM64 indirect operands.
* Expand `--help` message by listing options for `--isa`, `--syntax`, `--assembler`, and `--policy`.
* Fix bug resulting in skipped `.data` sections.
* Ubuntu 18 and gcc 7 are no longer supported.
* Default syntax in `assembler` mode changed to AT&T (i.e., `att`).

* Rename `elfSectionProperties` to `sectionProperties`, and remove
`peSectionProperties`.

* Use `.ascii` directive for partial strings.

* Add alternative PE assembler and linker commands.
* Use dedicated symbolic expression attributes.
* Add an option for assembler

* Added support for MIPS and ARM32
* Added support for MinGW for PE32+
* Fixed bad operand size in COMISS instructions
* Use PUBLIC entrypoint instead of EXPORT
* Handle linking to DRV files in PE32 binaries
* Change "isa" short option from -i to -I (-i is ir)
* Add special KUSER_SHARED_DATA symbol
* Handle ld-linux included as a required library
* Fix Intel syntax for vpgatherdd

* Add PE support.
* Remove null displacement offset warning.