Derive for our purposes something similar to what Linux has been having for
a long while.

v2 is merely a re-base, mainly targeted at reminding people that this series
is still pending review.

1: x86: introduce "brk" allocator
2: x86/EFI: replace ebmalloc()
3: xhci-dbc: use brk_alloc()

To reduce padding holes, .bss.page_aligned and per-CPU data may want moving
immediately ahead of __brk_start[]. Albeit then the tail of per-CPU data will
all be padding space; sadly the TSS wants/needs page-aligning for XPTI
purposes.

Jan