[llvm-readelf]Test a couple of corner-cases for --section-mapping

[jhenderson]

Summary

This patch adds two new tests for edge-case behaviour for --section-mapping, namely when there are no program headers, and when there are no section headers.

[mattd]

LGTM. Thanks for adding these.

llvm/trunk/test/tools/llvm-readobj/gnu-section-mapping-no-phdrs.test

+RUN: llvm-readelf --section-mapping %p/Inputs/trivial.obj.elf-x86-64 | FileCheck %s
+CHECK:      Section to Segment mapping:
+CHECK-NEXT:   Segment Sections...
+CHECK-NEXT:    None   .text .rela.text .data .bss .rodata.str1.1 .note.GNU-stack .shstrtab .symtab .strtab {{$}}
+CHECK-NOT: {{.}}

llvm/trunk/test/tools/llvm-readobj/gnu-section-mapping-no-shdrs.test

+RUN: llvm-objcopy --strip-sections %p/Inputs/phdrs-elf.exe-x86_64 %t.o
+RUN: llvm-readelf --section-mapping %t.o | FileCheck %s
+CHECK:      Section to Segment mapping:
+CHECK-NEXT:   Segment Sections...
+CHECK-NEXT:    00 {{$}}
+CHECK-NEXT:    01 {{$}}
+CHECK-NEXT:    02 {{$}}
+CHECK-NEXT:    03 {{$}}
+CHECK-NEXT:    04 {{$}}
+CHECK-NEXT:    05 {{$}}
+CHECK-NEXT:    06 {{$}}
+CHECK-NEXT:    07 {{$}}
+CHECK-NEXT:    08 {{$}}
+CHECK-NEXT:    09 {{$}}
+CHECK-NOT: {{.}}