[compiler-rt][profile] Fix various InstrProf tests on Solaris

Currently, several InstrProf tests `FAIL` on Solaris (both sparc and x86):

  Profile-i386 :: Posix/instrprof-visibility.cpp
  Profile-i386 :: instrprof-merging.cpp
  Profile-i386 :: instrprof-set-file-object-merging.c
  Profile-i386 :: instrprof-set-file-object.c

On sparc there's also

  Profile-sparc :: coverage_comments.cpp

The failure mode is always the same:

  error: /var/llvm/local-amd64/projects/compiler-rt/test/profile/Profile-i386/Posix/Output/instrprof-visibility.cpp.tmp: Failed to load coverage: Malformed coverage data

The error is from `llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp`
(`loadBinaryFormat`), l.926:

  InstrProfSymtab ProfileNames;
  std::vector<SectionRef> NamesSectionRefs = *NamesSection;
  if (NamesSectionRefs.size() != 1)
    return make_error<CoverageMapError>(coveragemap_error::malformed);

where .size() is 2 instead.

Looking at the executable, I find (with `elfdump -c -N __llvm_prf_names`):

  Section Header[15]:  sh_name: __llvm_prf_names
      sh_addr:      0x8053ca5       sh_flags:   [ SHF_ALLOC ]
      sh_size:      0x86            sh_type:    [ SHT_PROGBITS ]
      sh_offset:    0x3ca5          sh_entsize: 0
      sh_link:      0               sh_info:    0
      sh_addralign: 0x1

  Section Header[31]:  sh_name: __llvm_prf_names
      sh_addr:      0x8069998       sh_flags:   [ SHF_WRITE SHF_ALLOC ]
      sh_size:      0               sh_type:    [ SHT_PROGBITS ]
      sh_offset:    0x9998          sh_entsize: 0
      sh_link:      0               sh_info:    0
      sh_addralign: 0x1

Unlike GNU `ld` (which primarily operates on section names) the Solaris
linker, following the ELF spirit, only merges input sections into an output
section if both section name and section flags match, so two separate
sections are maintained.

The read-write one comes from `lib/clang/12.0.0/lib/sunos/libclang_rt.profile-i386.a(InstrProfilingPlatformLinux.c.o)`
while the read-only one is generated by
`llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp` (`InstrProfiling::emitNameData`)
at l.1004 where `isConstant = true`.

The easiest way to avoid the mismatch is to change the definition in
`compiler-rt/lib/profile/InstrProfilingPlatformLinux.c` to `const`.

This fixes all failures observed.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D85116