target/arm: Make SYS_HEAPINFO work with RAM that doesn't start at 0

target/arm: Make SYS_HEAPINFO work with RAM that doesn't start at 0

The semihosting SYS_HEAPINFO call is supposed to return an array
of four guest addresses:
 * base of heap memory
 * limit of heap memory
 * base of stack memory
 * limit of stack memory

Some semihosting programs (including those compiled to use the
'newlib' embedded C library) use this call to work out where they
should initialize themselves to.

QEMU's implementation when in system emulation mode is very
simplistic: we say that the heap starts halfway into RAM and
continues to the end of RAM, and the stack starts at the top of RAM
and works down to the bottom.  Unfortunately the code assumes that
the base address of RAM is at address 0, so on boards like 'virt'
where this is not true the addresses returned will all be wrong and
the guest application will usually crash.

Conveniently since all Arm boards call arm_load_kernel() we have the
base address of the main RAM block in the arm_boot_info struct which
is accessible via the CPU object.  Use this to return sensible values
from SYS_HEAPINFO.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20201119092346.32356-1-peter.maydell@linaro.org