[TESTS] Re-add issue20/issue26 tests

Summary:
Re-add tests removed because they used to depend on yaml2obj.
Rewrite them with an assembler (llvm-mc) and use the system linker to
produce a valid ELF as input to BOLT.

(cherry picked from FBD22323449)

bolt/test/X86/issue20.s

@@ -0,0 +1,53 @@
+# This reproduces issue 20 from our github repo
+#  "BOLT crashes when removing unreachable BBs that are a target
+#   in a JT"
+
+# REQUIRES: system-linux
+
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown \
+# RUN:   %s -o %t.o
+# RUN: %host_cc %t.o -o %t.exe
+# RUN: llvm-bolt %t.exe -relocs=0 -jump-tables=move -print-finalized \
+# RUN:    -o %t.out | FileCheck %s
+
+# CHECK: BOLT-INFO: UCE removed 0 blocks and 0 bytes of code.
+# CHECK: Binary Function "main"
+# CHECK:      .LFT{{.*}} (2 instructions, align : 1)
+# CHECK-NEXT:   CFI State : 0
+# CHECK-NEXT:     00000004:   andq
+# CHECK-NEXT:     00000008:   jmpq
+# CHECK-NEXT:   Successors: .Ltmp{{.*}}, .Ltmp{{.*}}, .Ltmp{{.*}}, .Ltmp{{.*}}
+
+
+  .text
+  .globl main
+  .type main, %function
+  .size main, .Lend1-main
+main:
+  xorq %rax, %rax
+  retq
+  andq $3, %rdi
+  jmpq *jumptbl(,%rdi,8)
+
+.Lbb1:
+  movl $0x1, %eax
+  jmp .Lexit
+.Lbb2:
+  movl $0x2, %eax
+  jmp .Lexit
+.Lbb3:
+  movl $0x3, %eax
+  jmp .Lexit
+.Lbb4:
+  movl $0x4, %eax
+.Lexit:
+  retq
+.Lend1:
+
+  .section .rodata
+  .globl jumptbl
+jumptbl:
+  .quad .Lbb1
+  .quad .Lbb2
+  .quad .Lbb3
+  .quad .Lbb4

bolt/test/X86/issue26.s

@@ -0,0 +1,56 @@
+# This reproduces issue 26 from our github repo
+#  BOLT fails with the following assertion:
+#    llvm/tools/llvm-bolt/src/BinaryFunction.cpp:2950: void llvm::bolt::BinaryFunction::postProcessBranches(): Assertion `validateCFG() && "invalid CFG"' failed.
+
+# REQUIRES: system-linux
+
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown \
+# RUN:   %s -o %t.o
+# RUN: %host_cc %t.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -relocs -print-cfg -o %t.out \
+# RUN:    | FileCheck %s
+
+# CHECK-NOT: BOLT-WARNING: CFG invalid in XYZ @ .LBB0
+
+# CHECK: Binary Function "XYZ"
+
+# CHECK: .Ltmp{{.*}} (1 instructions, align : 1)
+# CHECK-NEXT: Secondary Entry Point: FUNCat{{.*}}
+
+  .text
+  .globl XYZ
+  .type XYZ, %function
+  .size XYZ, .Lend1-XYZ
+XYZ:
+  movl %fs:-0x350, %eax
+  cmpl %eax, %edi
+  jne .L1
+
+  cmp %rdx, (%rsi)
+  jne .L2
+
+  movq %rcx, (%rsi)
+.L1:
+  retq
+
+.L2:
+  movl $0xffffffff, %eax
+  retq
+.Lend1:
+
+  .globl FUNC
+  .type FUNC, %function
+  .size FUNC, .Lend - FUNC
+FUNC:
+  cmpq %rdi, %rsi
+  je .L1
+  retq
+.Lend:
+
+  .globl main
+  .type main, %function
+  .size main, .Lend2 - main
+main:
+  xorq %rax, %rax
+  retq
+.Lend2:

bolt/test/X86/pre-aggregated-perf.test

@@ -7,7 +7,7 @@
 #  blarge is the binary for "basicmath large inputs" taken from Mibench.
 
 # Currently failing in MacOS / generating different hash for usqrt
-REQUIRES: Linux
+REQUIRES: system-linux
 
 RUN: yaml2obj %p/Inputs/blarge.yaml &> %t.exe
 RUN: perf2bolt %t.exe -o %t -pa -p %p/Inputs/pre-aggregated.txt -w %t.new