firmware/seabios: fix build on systems with non GNU toolchains
SeaBIOS requires gcc and GNU ld in order to build, so allow setting
SEABIOSCC and SEABIOSLD by the caller when building in order to pass
the path to the compiler and linker that should be used when building
Note that the LD32BIT-y variable was used by FreeBSD builds and is no
longer needed due to the ability to select the LD to use for SeaBIOS
builds, so remove it.