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 CodeGen option "-arm-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 option.
* 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.
llvm-svn: 289784
Eric Christopher