xen/arm32: heap: Rework adr_l so it doesn't rely on where Xen is loaded

xen/arm32: heap: Rework adr_l so it doesn't rely on where Xen is loaded

At the moment, the macro addr_l needs to know whether the caller
is running with the MMU on. This is fine today because there are
only two possible cases:
 1) MMU off
 2) MMU on and linked to the virtual address

This is still cumbersome to use for the developer as they need
to know if the MMU is on.

Thankfully, Linux developpers came up with a great way to allow
adr_l to work within the range +/- 4GB of PC by emitting a PC-relative
reference [1].

Re-use the same approach on Arm and drop the parameter 'mmu'.

[1] 0b1674638a5c ("ARM: assembler: introduce adr_l, ldr_l and str_l macros")

Signed-off-by: Julien Grall <jgrall@amazon.com>