Fix gdb issue where the i-cache was not being updated when a breakpoint

Fix gdb issue where the i-cache was not being updated when a breakpoint
was written into a user's address space. The fix is to modify uiomove_fromphys
to sync the icache when an executable user-space page is written into.

Alan Cox suggested that there should probably be a higher-level interface
to this in the ptrace code, but agreed that this is an OK short-term solution.

Files changed:

pmap.h - declaration of pmap_page_executable()
pmap_dispatch.c - pass through the page_executable call to the mmu object
mmu_oea.c - implement the page_executable method by examining the PTE_EXEC
 field in the vm_page_t
uio_machdep.c - in uiomove_fromphys(), if the op was a UIO_WRITE to user-space,
 and if the page is executable, sync the icache since this is at the least
 a breakpoint-write from gdb.

Reported by:	marcel
Tested by:	marcel, grehan on g3+g4
Discussed with:	alc
MFC after:	2 weeks