Fix mips-related build breakage.

Bug: http://b/34396220
Test: Builds successfully.

MIPS targets predefine _mips to 1, which will conflict with the _mips
variable name that was being used in the HAL here. Rename it to _amips
instead, so that it won't conflict.

Change-Id: Id13f34b740959852dff600259e3405eb72d0983a