Refine constraint "Bk" to define_special_memory_constraint.

For below pattern, RA may still allocate r162 as v/k register, try to
reload for address with leaq __libc_tsd_CTYPE_B@gottpoff(%rip), %rsi
which result a linker error.

(set (reg:DI 162)
     (mem/u/c:DI
       (const:DI (unspec:DI
		 [(symbol_ref:DI ("a") [flags 0x60]  <var_decl 0x7f621f6e1c60 a>)]
		 UNSPEC_GOTNTPOFF))

Quote from H.J for why linker issue an error.
>What do these do:
>
>        leaq    __libc_tsd_CTYPE_B@gottpoff(%rip), %rax
>        vmovq   (%rax), %xmm0
>
>From x86-64 TLS psABI:
>
>The assembler generates for the x@gottpoff(%rip) expressions a R X86
>64 GOTTPOFF relocation for the symbol x which requests the linker to
>generate a GOT entry with a R X86 64 TPOFF64 relocation. The offset of
>the GOT entry relative to the end of the instruction is then used in
>the instruction. The R X86 64 TPOFF64 relocation is pro- cessed at
>program startup time by the dynamic linker by looking up the symbol x
>in the modules loaded at that point. The offset is written in the GOT
>entry and later loaded by the addq instruction.
>
>The above code sequence looks wrong to me.

gcc/ChangeLog:

	PR target/116043
	* config/i386/constraints.md (Bk): Refine to
	define_special_memory_constraint.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr116043.c: New test.

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