Accorading to GCC and Clang documents, march=mipsel/mips64el doesn't
really exist. So we use real CPU arch instead, and apply '-EL' flag
to indicate endianess. Also, to deal with link failure, we add '-mxgot'
to increase GOT size.

Change-Id: I21706da3423412c968f6e80a9d21f5266255a0cf
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41028Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Ben Clayton <bclayton@google.com>