make type of st_dev explicitly dev_t in x86_64 stat.h

otherwise it's unclear that it's correct. aside from that, it makes
for a gratuitous difference between the x86_64 header and the upcoming
x32 header.
remove size suffix in x86_64 __pthread_self asm

the operand size is unnecessary, since the assembler knows it from the
destination register size. removing the suffix makes it so the same
code should work for x32.
remove gratuitous temp vars, casts, and suffixes in x86_64 atomic.h

aside from general cleanup, this should allow the identical atomic.h
file to be used for the upcoming x32 port.