Add option to build Chapel with mimalloc as either the host or target allocator

[jabraham17]

Adds the ability to set CHPL_HOST_MEM and/or CHPL_TARGET_MEM to mimalloc.

This PR only provides the option to use mimalloc, it does not change the default from jemalloc

Performance comparisons for CHPL_TARGET_MEM on linux64

test/performance/memory/microMemoryAllocation.chpl

Using --trials=2_000_000

• jemalloc: 1.38
• mimalloc: 7.79

test/studies/shootout/binary-trees/binarytrees-inner.chpl

Using --n=21

• jemalloc: 2.274
• mimalloc: 0.984

Performance comparisons for CHPL_HOST_MEM on linux64

Compiling examples/hello.chpl

• jemalloc: 6.0s
• mimalloc: 5.1s

Compiling Arkouda

• jemalloc: TODO
• mimalloc: TODO

Major changes in this PR

• Add support to use mimalloc from $CHPL_HOME/third-party/mimalloc or from the system
• Add a bundled mimalloc 2.1.7

Correctness testing

• paratest with/without gasnet with CHPL_TARGET_MEM=mimalloc on linux64
• paratest with/without gasnet with CHPL_HOST_MEM=mimalloc on linux64
• make check with/without gasnet with CHPL_TARGET_MEM=mimalloc on MacOS
• make check with/without gasnet with CHPL_HOST_MEM=mimalloc on MacOS
• Test that fixed heap config with SS11 gives proper errors

Future work:

• enable fixed heap configs
• enable asan builds