[MIPS] Update llvm 7.0 gn build file for mipsel
[MIPS] Update llvm 7.0 gn build file for mipsel

This enables LLVM reactor backend to be used for mipsel
in Chromium, though Subzero remains the default option.

Bug: b/117854176
Change-Id: I8c66017cd3a966202b61f8cbd65e065e3512e1e5
Reviewed-on: https://swiftshader-review.googlesource.com/c/22008
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Gordana Cmiljanovic <f1887b663daaed5aaf2a0a1d3e414203178e5673@mips.com>

google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader