Fix build of hyperv with base gcc on i386

Fix build of hyperv with base gcc on i386

Summary:
Base gcc fails to compile `sys/dev/hyperv/pcib/vmbus_pcib.c` for i386,
with the following -Werror warnings:

cc1: warnings being treated as errors
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c: In function 'new_pcichild_device':
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c:567: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c: In function 'vmbus_pcib_on_channel_callback':
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c:940: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c: In function 'hv_pci_protocol_negotiation':
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c:1012: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c: In function 'hv_pci_enter_d0':
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c:1073: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c: In function 'hv_send_resources_allocated':
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c:1125: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c: In function 'vmbus_pcib_map_msi':
/usr/src/sys/dev/hyperv/pcib/vmbus_pcib.c:1730: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

This is because on i386, several casts from `uint64_t` to a pointer
reduce the value from 64 bit to 32 bit.

For gcc, this can be fixed by an intermediate cast to uintptr_t. Note
that I am assuming the incoming values will always fit into 32 bit!

Differential Revision: https://reviews.freebsd.org/D15753
MFC after:	3 days