Stub out example CFI that generates a common compact unwind entry table.

This is only a temporary solution, as relying the assembler producing
specific output given a set of CFI input directives is 1) annoying to
work with, and 2) likely to fail due to future toolchain changes.

Resources/Tests/PLCrashAsyncCompactUnwindEncodingTests/example.S

@@ -1,9 +1,41 @@
 #if defined(__x86_64__)
+//  This triggers common encoding handling with _main
+.globl _main_common
+_main_common:
+	.cfi_startproc
+	pushq	%rbp
+	.cfi_def_cfa_offset 16
+	.cfi_offset %rbp, -16
+	movq	%rsp, %rbp
+	.cfi_def_cfa_register %rbp
+
+	movl	$5, %eax
+	popq	%rbp
+	ret
+	.cfi_endproc
+
+// This hack forces the use of a common encoding
+// by preventing the use of an entry range that includes
+// both _main_common and _main
+.globl _main_noncommon
+_main_noncommon:
+	.cfi_startproc
+	.cfi_def_cfa_offset 32
+	nop
+	ret
+	.cfi_endproc
+
 .globl _main
 _main:
 	.cfi_startproc
-	// TODO
+	pushq	%rbp
+	.cfi_def_cfa_offset 16
+	.cfi_offset %rbp, -16
+	movq	%rsp, %rbp
+	.cfi_def_cfa_register %rbp
+
 	movl	$5, %eax
+	popq	%rbp
 	ret
 	.cfi_endproc
 #elif defined(__i386__)