external-csl-toolchain: support mips/ppc/arm

external-csl-toolchain: support mips/ppc/arm

- Rather than adjusting TARGET_VENDOR, which results in our TARGET_SYS
  matching the external toolchain, override TARGET_PREFIX to match external,
  and leave TARGET_SYS alone
- Grab the optimized files out of the toolchain if available
- Create a symlink in sysroot to ensure the sysroot layout matches toolchain
  expectations (optimized files in a subdir)

(From OE-Core rev: a37298eb3421a44e88ec5a66b2fc5305ab18f453)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>