x86: embed register names in reg_entry

Register names are (including their nul terminators) on average almost 4
bytes long. Otoh no register name is longer than 7 bytes. Hence even for
32-bit builds using a pointer is only slightly more space efficient than
embedding the strings. A level of indirection can be also avoided by
embedding the names as an array of 8 characters directly in the struct,
and the number of base relocations in PIE builds of gas goes down as
well.