This PR adds basic support for the MIPS R5900 CPU, the Emotion Engine
processor used in the PlayStation 2.
**LLVM changes:**
- Add r5900 CPU definition (with soft float support for now)
- Disable instructions not supported by r5900 (64-bit multiply/divide,
LL/SC atomics, COP3)
- Add r5900 specific short loop delay slot fix (hardware errata
workaround)
- Set ISA extension `AFL_EXT_5900` in ELF flags for proper ABI
identification
**Clang changes:**
- Add r5900 as a valid CPU target for `-mcpu=r5900`
- Add r5900 to CPU test coverage
f4b593a792