jemalloc: allow on MIPS64

jemalloc uses architecture #ifdefs to determine LG_QUANTUM and gives an
error when an unsupported architecture is used.
For this reason, Buildroot commit 3baf996c
introduced BR2_PACKAGE_JEMALLOC_ARCH_SUPPORTS.

In the jemalloc sources, 'mips' is checked via '__mips__' which is set both
for 32-bit as 64-bit MIPS (including MIPS64 n32).
However, the Buildroot arch selection only includes 32-bit MIPS via BR2_mips
and BR2_mipsel.

Update the arch selection to support MIPS64.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>