Enable RIP-relative addressing for JIT access to class statics on x64 non-windows

[AndyAyersMS]

getRelocTypeHint always returns false on Linux, so the JIT always ends up using a full literal 64 bit displacement to access static fields. This makes static field access relatively more expensive, both in terms of code size and instruction count (especially so since the JIT does not CSE large literals). For instance, to increment a static and return the incremented value:

;; Windows
G_M28232_IG02:
       8B058E0ADDFF         mov      eax, dword ptr [reloc classVar[0xea02f330]]
       8D5001               lea      edx, [rax+1]
       8915850ADDFF         mov      dword ptr [reloc classVar[0xea02f330]], edx

G_M28232_IG03:
       C3                   ret      

;; Linux
G_M5560_IG02:
       48B834474066A17F0000 mov      rax, 0x7FA166404734
       8B00                 mov      eax, dword ptr [rax]
       8D7801               lea      edi, [rax+1]
       48BE34474066A17F0000 mov      rsi, 0x7FA166404734
       893E                 mov      dword ptr [rsi], edi

G_M5560_IG03:
       C3                   ret      

category:cq
theme:runtime
skill-level:expert
cost:medium

[briansull]

PR dotnet/coreclr#12831 should fix this.

[EgorBo]

Just checked - it's already done e.g.

private static int f;

[MethodImpl(MethodImplOptions.NoInlining)]
static int Test() => f;

; Assembly listing for method Program:Test():int
; Emitting BLENDED_CODE for X64 with AVX512 - Unix
; Tier-1 compilation
; optimized code
; rsp based frame
; partially interruptible
; No PGO data

G_M000_IG01:                ;; offset=0000H

G_M000_IG02:                ;; offset=0000H
       mov      eax, dword ptr [(reloc 0x7f2bc5557d58)]

G_M000_IG03:                ;; offset=0006H
       ret

; Total bytes of code 7

checked on ubuntu-x64