Disable -Wdangling-pointer for the kernel for GCC 12.

Some of the warnings raised in the kernel seem to be outright bugs in
the compiler (e.g. the cases in ata_xpt.c and scsi_xpt.c). Other
cases are not fatal and it didn't seem to find any legitimate bugs in
the kernel.

Differential Revision: https://reviews.freebsd.org/D37629