[Courgette] Fix AssemblyProgram parsing for ELF-ARM.

This fixes two problems:
(A) In DisassemblerElf32, RVAToFileOffset() used Program Segment Header,
  but FileOffsetToRVA() used Section Header. For consistency, both
  should use the same one. We choose the latter (for now).
(B) Even if a section has sh_type=SHT_PROGBITS, it can still have
  sh_addr=0. Extracting Rel32 address from these sections would add
  overlapping RVA chaos, and so should be avoided.

Also using elf-arm7 in a unit test. The test fails before the fix and
passes after.

BUG= 579206

Review URL: https://codereview.chromium.org/1658463002

Cr-Commit-Position: refs/heads/master@{#373639}

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