x86/boot/compressed: Avoid duplicate malloc() implementations

The early malloc() and free() implementation in include/linux/decompress/mm.h
(which is also included by the static decompressors) is static. This is
fine when the only thing interested in using malloc() is the decompression
code, but the x86 early boot environment may use malloc() in a couple places,
leading to a potential collision when the static copies of the available
memory region ("malloc_ptr") gets reset to the global "free_mem_ptr" value.
As it happened, the existing usage pattern was accidentally safe because each
user did 1 malloc() and 1 free() before returning and were not nested:

extract_kernel() (misc.c)
	choose_random_location() (kaslr.c)
		mem_avoid_init()
			handle_mem_options()
				malloc()
				...
				free()
	...
	parse_elf() (misc.c)
		malloc()
		...
		free()

Once the future FGKASLR series is added, however, it will insert
additional malloc() calls local to fgkaslr.c in the middle of
parse_elf()'s malloc()/free() pair:

	...