musl: mips: add vdso support

musl: mips: add vdso support

vdso support is available on mips starting with kernel 4.4, see kernel
commit a7f4df4e21 "MIPS: VDSO: Add implementations of gettimeofday()
and clock_gettime()" for details.

These are my micro benchmark results for 1.000.000 calls to
clock_gettime(CLOCK_MONOTONIC, &tp)

without vdso:
root@OpenWrt:/# time ./vdso-test
real 0m 0.95s
user 0m 0.24s
sys 0m 0.70s

with vdso:
root@OpenWrt:/# time /usr/bin/vdso-test
real 0m 0.35s
user 0m 0.34s
sys 0m 0.00s

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48466 3c298f89-4303-0410-b956-a3cf2f4a3e73