Fix support for amd64 x32 ABI

[peace-maker]

x32 detection was broken since fbf2727
There is special handling in the plt emulation for this, which cannot be reached anymore.

pwntools/pwnlib/elf/plt.py

Lines 80 to 83 in 65f9d57

	# x32 uses 64-bit instructions, just restricts itself to a 32-bit
	# address space.
	if elf.arch == 'amd64' and elf.bits == 32:
	emulation_bits = 64

Setting context.arch while loading a x32 ELF caused an exception in the LocalContext.__enter__ function, which caused the context not getting reset while leaving the contextmanager. This causes the behavior seen in #2304, where the log_level is reset to the default INFO and not reset to the old one after leaving the context. (Python context managers not to be confused with pwntools context class)

pwntools/pwnlib/elf/elf.py

Lines 278 to 285 in 65f9d57

	# Is this a native binary? Should we be checking QEMU?
	try:
	with context.local(arch=self.arch):
	#: Whether this ELF should be able to run natively
	self.native = context.native
	except AttributeError:
	# The architecture may not be supported in pwntools
	self.native = False

Fixes #2304

[Arusekk]

Do we want to add a small x32 binary for tests? I think we can do without it, but just curious what's your opinion.

Anyway, just as a note, I think it will be nice to refer to the faulty commit in the message of the merged commit.

[peace-maker]

I've compiled a small program with the different flags to test PLT emulation on x32 too.

gcc test.c -o test-x32 -mx32 -no-pie -Wl,-z,norelro -fstack-protector-all
gcc test.c -o test-x32-pie -mx32 -pie -Wl,-z,norelro -fstack-protector-all
gcc test.c -o test-x32-relro -mx32 -no-pie -Wl,-z,relro,-z,now -fstack-protector-all
gcc test.c -o test-x32-relro-pie -mx32 -pie -Wl,-z,relro,-z,now -fstack-protector-all