[ARM] Implement execute-only support in CodeGen
This implements execute-only support for ARM code generation, which
prevents the compiler from generating data accesses to code sections.
The following changes are involved:
- Add the SubtargetFeature "execute-only" to the ARM code generator.
- Add the clang flag "-mexecute-only" as well as the GCC-compatible alias "-mpure-code" to enable this feature.
- When enabled, literal pools are replaced with MOVW/MOVT instructions, with VMOV used in addition for floating-point literals. As the MOVT instruction is required, execute-only support is only available in Thumb mode for targets supporting ARMv8-M baseline or Thumb2.
- Jump tables are placed in data sections when in execute-only mode.
- The execute-only text section is assigned section ID 0, and is marked as unreadable with the SHF_ARM_PURECODE flag with symbol 'y'. This also overrides selection of ELF sections for globals.