jit-dasm-pmi - add explicit GC

On Linux (x64 and arm32) the following observed when there is a race between JIT compiling method and the finalizer thread (and the finalizer also got jitted).

```
@@ -120401,79 +120401,79 @@ G_M40565_IG02:
movs r4, 0
movs r2, 0
str r2, [sp+0x0c]
- adr r2, G_M40565_IG04
+ adr r2, G_M40566_IG04
str r2, [sp+0x14]
movs r2, 0
strb r2, [r5+8]

-G_M40565_IG03:
+G_M40566_IG03:
blx r3

-G_M40565_IG04:
+G_M40566_IG04:
movs r3, 1
strb r3, [r5+8]
movw r3, 0xd1ff
movt r3, 0xd1ff
ldr r3, [r3]
cmp r3, 0
- beq SHORT G_M40565_IG05
+ beq SHORT G_M40566_IG05
movw r12, 0xd1ff
movt r12, 0xd1ff
-; Assem b l y list i ng for blxmethod System.Gen2GcCallback:Finalize():this
-; Emitting BLENDED_CODE for generic ARM CPU
-; optimized code
-; r11 r12based fr ame
- ; f/ully/ int errupCORINFO_HELP_STOP_FOR_GCtibl
-e
+ blx r12 // CORINFO_HELP_STOP_FOR_GC

-G_M40565_IG05:
- ; F inalmov sloc al va riabl e assignments
-r3;
-, 0
- str ; r3, [V00 thissp+ 0x[V14]00,T00] ( 7, 6 ) ref
- ->
-G_M[sp40565+_0xIG04]06 :
- do-not -enre g [ H]mov st h i s class-hndr3
-; , 1
-V01 loc0 [V 01,T06] ( 3, 3 ) ref -> strb r1 class- hnd
-;# r3, [r5+V02 OutArgs8 ][V
-02 ] ( 1 , 1 )ldr lclBlk ( 0) r3[, sp[+0xsp00+] 0x
-08]
- str r3, [r5;+* 12]
- V03 tmp1 [V03 ] ( 0, 0 ) ref -> addz e r o -r ef sp ,cla ss-h36
- pop {nr4,dr5
-,;r6 , r7,r8,r9V04 tmp2, r10[V,04r11,T,02]pc} (
- 2,
- 4 ) ; Tot byrefa l b y-tes o>f code r0 100 , pr o l og
-size ; 14 for method V05 tmp3DomainNeutralILStubClass:IL_STUB_PInvoke(int):int
- ;[V =============05======,=====T===============03] ( 2, 4 ) int -> ===== r3= === ==== ==== ====
- ld-addr-op
```

It was suggested by @AndyAyersMS and @BruceForstall to explicitly call GC (e.g. before PrepareMethod) so GC happens more predictably. It worked for me but this makes jit-dasm-pmi much slower - so it makes sense to add this as an option which can be specified in command line (say, `--force_gc_collect`)?

I have never seen this happening on Windows though.