Added perlin noise generation benchmark

Adapted from https://github.com/nothings/stb, released in the public domain.
On a note not related to this pull request; this sort of thing will need to be check for in other benchmarks (eg here https://github.com/mageec/beebs/blob/569bcbe86b94c97de085dc0d4af1af9230a3274b/src/crc32/crc_32.c#L72) because I wasn't checking for it when I ported them to ARM.
Some of the "check" arrays are large but I think they're all in main() anyway and the compiler is bundling them with instructions (at least it did when I just tested int_matmult ).