The `--disable-jemalloc` configure option has a failure mode where it will
create a distribution that is not compatible with other compilers. For example
the nightly for Linux will assume that it will link to jemalloc by default as
an allocator for executable crates. If, however, a standard library is used
which was built via `./configure --disable-jemalloc` then this will fail
because the jemalloc crate wasn't built.
While this seems somewhat reasonable as a niche situation, the same mechanism is
used for disabling jemalloc for platforms that just don't support it. For
example if the rumprun target is compiled then the sibiling Linux target *also*
doesn't have jemalloc. This is currently a problem for our cross-build nightlies
which build many targets. If rumprun is also built, it will disable jemalloc for
all targets, which isn't desired.
This commit moves the platform-specific disabling of jemalloc as hardcoded logic
into the makefiles that is scoped per-platform. This way when configuring
multiple targets **without the `--disable-jemalloc` option specified** all
targets will get jemalloc as they should.
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