Use grub-macho2img when compiling with Apple's CC for PCBIOS machine
* conf/common.rmk (bin_UTILITIES): add (on false on condition)
grub-macho2img
(CLEANFILES): add grub-macho2img
(grub_macho2img_SOURCES): new variable
* kern/i386/pc/startup.S (bss_start): new variable
(bss_end): likewise
* genmk.rb: use grub-macho2img for *.img when compiled with Apple's CC
* util/grub-macho2img.c: new file