MIPS: 32/64-bit VDSO support

File libc/SYSCALLS.TXT is updated to generate bionic's system call wrappers
for clock_gettime() & gettimeofday() that will be called if kernel vdso
implementations fail to execute.
The system call wrappers are generated using a python script gensyscalls.py.

Since all architectures support vdso now, there is no more need for conditional
statements regarding supported architectures in libc/bionic/vdso.cpp &
libc/private/bionic_vdso.h files.

Test: builds
Change-Id: I7213f29c179a7929851499d78a72900638ae861a
Signed-off-by: Goran Ferenc <goran.ferenc@imgtec.com>