[X86] Emit RIP-relative access to local function in PIC medium code model

Currently, the medium code model for x86_64 emits position-dependent relocations (R_X86_64_64) for local functions, regardless of PIC or no-PIC mode. (This means generically that code compiled with the medium model cannot be linked into a position-independent executable.)

Example:

```
static int g(int n) {
  return 2 * n + 3;
}

void f(int(**p)(int)) {
  *p = g;
}
```

This results in:

```
Disassembly of section .text:

0000000000000000 <f>:
       0: 48 b8 00 00 00 00 00 00 00 00	movabs	rax, 0x0
       a: 48 89 07                     	mov	qword ptr [rdi], rax
       d: c3                           	ret
```

```
Relocation section '.rela.text' at offset 0xf0 contains 1 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000000002  0000000200000001 R_X86_64_64            0000000000000000 .text + 10
```

This patch changes the behaviour to unconditionally emit a RIP-relative access, both in PIC and non-PIC mode. This fixes PIC mode, and is perhaps an improvement in non-PIC mode, too, since it results in a shorter instruction. A 32-bit relocation should suffice since the medium memory model demands that all code fit within 2GiB.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D140593