Don't bail out of all.sh if the OS isn't Linux. We only expect
everything to pass on a recent Linux x86_64, but it's useful to call
all.sh to run some components on any platform.
In all.sh, always run both MemorySanitizer and Valgrind. Valgrind is
slower than ASan and MSan but finds some things that they don't.
Run MSan unconditionally, not just on Linux/x86_64. MSan is supported
on some other OSes and CPUs these days.
Use `all.sh --except test_memsan` if you want to omit MSan because it
isn't supported on your platform. Use `all.sh --except test_memcheck`
if you want to omit Valgrind because it's too slow.
Make the test scripts more portable (tested on FreeBSD): don't insist
on GNU sed, and recognize amd64 as well as x86_64 for `uname -m`. The
`make` utility must still be GNU make.
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