x86, boot: align the .bss section in the decompressor

Aligning the .bss section makes it trivial to use large operation
sizes for moving the initialized sections and clearing the .bss.
The alignment chosen (L1 cache) is somewhat arbitrary, but should be
large enough to avoid all known performance traps and small enough to
not cause troubles.

[ Impact: trivial performance enhancement, future patch prep	]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>