Makefile: fix cross compile

[SupervisedThinking]

Fixes cross compilation. When you build amiberry in a build root that has special build tools for specific targets e.g. https://github.com/SupervisedThinking/LibreELEC-RR/blob/master-rr/packages/supervisedthinking/emulation/standalone/amiberry/package.mk build will fail because host as will be used:

as: unrecognized option '-mfloat-abi=hard'
make: *** [Makefile:447: src/osdep/neon_helper.o] Error 1
make: *** Waiting for unfinished jobs....

Changes proposed in this pull request:
only define AS if AS wasn't already defined so change AS = as to AS ?= as so the toolchain as will be used instead of host as

@midwan

[midwan]

Thanks!