semihosting: Support SYS_HEAPINFO when env->boot_info is not set

semihosting: Support SYS_HEAPINFO when env->boot_info is not set

env->boot_info is only set in some ARM startup paths, so we cannot
rely on it to support the SYS_HEAPINFO semihosting function. When not
available, fallback to finding a RAM memory region containing the
current stack and use the base of that.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210107170717.2098982-5-keithp@keithp.com>
Message-Id: <20210108224256.2321-16-alex.bennee@linaro.org>