misc: respect standard __ASSEMBLER__ define in CPU headers
The JZ47xx and S5L87xx processor families used their own
special defines (__ASSEMBLY__ and ASM respectively) in
their CPU headers to check if they were included from an
assembly source file.
For GCC the standard seems to be __ASSEMBLER__, so check
for that instead and remove the non-standard symbols.
Being more consistent across platforms makes it easier to
include cpu.h from cross-platform files (eg. plugin.lds).
Change-Id: I282930cad34e1a2ff18166f3b4338548b34f4a49