Update jemalloc to version 5.2.1

[vangyzen]

Summary

Revert r354606 to restore r354605.

Apply one line from jemalloc commit d01b425e5d1e1 in hash_x86_128()
to fix the build with gcc, which only allows a fallthrough attribute
to appear before a case or default label.

Test Plan

• universe worlds built with llvm
• amd64 libc built with gcc 6.4.0, 6.5.0, and 9.2.0
• Isilon OneFS has run with this for 3 months

[vangyzen]

contrib/jemalloc/include/jemalloc/internal/hash.h

202

Here is the one line I mentioned.

[vangyzen]

@acidburn0.pt_gmail.com: Are you Jason Evans, author of jemalloc?

[lwhsu]

In D24522#539355, @vangyzen wrote:

@acidburn0.pt_gmail.com: Are you Jason Evans, author of jemalloc?

Looks like not, I guess it would be best to send him a mail directly.