Released SDCC 4.5.0 and 4.6.0 miscompile some of our code in the mcs51
register allocator (SDCC bugs 4153 and 4154, fixed in trunk r17025 and
r17039). The first make now downloads the 4.6.0 sources, checks their
sha256, applies the two fixes from sdcc/ and installs the compiler into
toolchain/, named after a checksum of sdcc/, so a changed patch builds a
new one. make CC=sdcc still uses an installed compiler.
