Remove mips-specific arch flags
Remove mips-specific arch flags

Most of these flags are already in the default //build setup
in build/config/compiler/BUILD.gn, config("compiler_cpu_abi").
`-mxgot` isn't -- if that flag is needed, it should probably
be in the upstream gn build files. If it's needed just for
swiftshader we can add it back with a comment that says why
the flag is swiftshader-specific.

Change-Id: Ia1a3b9770119f575fe3fec9fc18237c89b935800
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55629
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Commit-Queue: Nico Weber <c0f02d98506190746d1cdcb01be0f37b51e27077@chromium.org>
Tested-by: Nico Weber <c0f02d98506190746d1cdcb01be0f37b51e27077@chromium.org>

google/swiftshader,google/swiftshader,google/swiftshader