x86/ELF: fix .tfloat output with hex input

The ELF psABI-s are quite clear here: On 32-bit the data type is 12
bytes long (with 2 bytes of trailing padding), while on 64-bit it is 16
bytes long (with 6 bytes of padding). Make hex_float() capable of
handling such padding.

Note that this brings the emitted data size of .dc.x / .dcb.x in line
also for non-ELF targets; so far they were different depending on input
format (dec vs hex).

Extend the existing x86 testcases.