rename upsampling_mips32.c to yuv_mips32.c

matches yuv_sse2 added in;
bdfeeba dsp/yuv: move sse2 functions to yuv_sse2.c

Change-Id: I84f7d7858ca6851c956e8366a7c76b45070dcbc3
configure: test for -msse2

+ add a WEBP_HAVE_SSE2 to dsp.h

not all 32-bit toolchain configurations will have sse2 enabled by
default

Change-Id: I7c675e511581f93cf55c79f960fa7efa2df4987e
Merge changes I7c675e51,I84f7d785

* changes:
  configure: test for -msse2
  rename upsampling_mips32.c to yuv_mips32.c