x86/ELF: fix .tfloat output

x86/ELF: fix .tfloat output

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 ieee_md_atof() capable of
handling such padding, and specify the needed padding for x86 (leaving
non-ELF targets alone for now). Split the existing x86 testcase.