porting jpeg_mem_src() from jpeg lib v8c

A new function jpeg_mem_src() is ported from jpeg lib v8c to
support decode/encode in memory.  The source code is coming from
merged of Change-Id: I4d7f81fbb9a9e0cecaadd1855fc9f34296ba3eae
http://www.ijg.org/files/jpegsrc.v8c.tar.gz

Change-Id: Ibbc86ae6d5d041b7018b640395f8c7399de76acb
VeNum optimizations

jpeg-VeNum-optimization-of-IDCT-YCbCr-to-RGB-color-c
Preserve registers D8-D15 when using VFP as specified by
AAPCS v2.03

Change-Id: I2bd83a62f11b73f02217f9581e39de40b1dfcaa8