x86: fix Solaris testsuite failures

For one 0afc614c ("x86: Warn .insn instruction with length > 15
bytes") introduced a .insn use involving a slash; such tests need to
have --divide passed to gas.

And then 5bc71c2a ("x86-64: Add R_X86_64_CODE_6_GOTTPOFF") broke
BFD_RELOC_X86_64_GOTTPOFF conversion to R_X86_64_CODE_4_GOTTPOFF, by
adding respective code in a section guarded by
generate_relax_relocations (the case of that not being required there
was limited to 32-bit object files). Re-arrange that block of code to
check generate_relax_relocations later.