Update jemalloc to version 5.2.1

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.

Submitted by: jasone in r354605
Discussed with: jasone
Reviewed by: bdrewery
MFC after: never, due to gcc 4.2.1
Relnotes: yes
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D24522

Revert r354605: Update jemalloc to version 5.2.1.

Compilation fails for non-llvm-based platforms.

Update jemalloc to version 5.2.1.