elfutils: enable building with MIPS16

elfutils: enable building with MIPS16

Building with MIPS16 was disabled in 2013 due to an issue with GCC TLS:
https://dev.archive.openwrt.org/ticket/13572. But after the problematic
GCC version was retired, this change wasn't revisited.

Re-enable MIPS16 builds to reduce average elfutils library sizes ~10%.
This was compile-tested on malta/mips32be and malta/mips32le, and linked
with iproute2 for run-testing. Package sizes follow:

Library  MIPS16:=0  MIPS16:=1
-------  ---------  ---------
libelf1    43217      37492
libasm1    12481      11658
libdw1    229723     205793

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>