musl: Add mimalloc as a possible malloc implementation

Add mimalloc as an alternative malloc implementation and use it. This closes glibc's malloc performance advantage over musl and in some situations makes more efficient use of memory.

In testing, mimalloc outperformed mallocng, jemalloc and dlmalloc. Not yet tested: Bionic malloc

Signed-off-by: Bernhard Rosenkränzer bernhard.rosenkraenzer.ext@huawei.com