LLVM JIT: Recompile all widths of aligned loads

The new opcodes are LB, LBU, LH, LHU, LWU and LD.
They have minimal code duplication among each other and with LW.

They are currently implemented identically to the cached interpreter's
version, so a memory accessor function is still called if the load
comes from RDRAM.

Three new global variables are needed: readmemb, readmemh and readmemd,
the arrays containing memory accessors for 8-bit, 16-bit and 64-bit
loads.