This adds the option FULLY_STATIC to create fully-static executables.

To create portable, fully-static release executables on Linux, we need
to use musl instead of glibc.  Static executables from glibc are not
portable.

The popular musl-gcc wrapper does not support C++, so instead we use a
full musl cross-compiler toolchain in the build workflow.

To build FULLY_STATIC, the user must point to the appropriate
cross-compiler, as we do in the workflow.  On systems where musl is
the native libc (such as Alpine Linux), this is not necessary.

I have also read that musl's allocator is not very fast in
multi-threaded applications.  So when FULLY_STATIC is enabled, we will
also enable mimalloc, a replacement allocator that is very fast.

I tested a very basic packaging command to compare speeds of dynamic
glibc, static musl, and static musl+mimalloc:

dynamic glibc:
runs: 2.527, 2.798, 2.703, 2.756, 2.959
avg = 2.749, std dev = 0.156s

static musl:
runs: 2.813, 2.920, 3.129, 3.003, 2.738
avg = 2.921s, std dev = 0.154s

static musl+mimalloc:
runs: 2.291, 2.034, 2.415, 2.303, 2.265
avg = 2.262s, std dev = 0.140s

The mimalloc build is 82% faster than musl default allocator, 77%
faster than glibc, and has more consistent runtime characteristics
(lower standard deviation).

ldd does not return exit code of 1 any more for static, and "file" reports dynamic on arm64 even when it is not.  So now we look at the output of ldd instead.