Fix -Wconversion warnings in ZGC code

There are two warnings in zPage.inline.hpp that are reported a lot (1226) times because of inclusion.

src/hotspot/share/gc/z/zPage.inline.hpp: In member function 'uint8_t ZPage::numa_id()':
src/hotspot/share/gc/z/zPage.inline.hpp:183:32: warning: conversion from 'uint32_t' {aka 'unsigned int'} to 'uint8_t' {aka 'unsigned char'} may change value [-Wconversion]
  183 | _numa_id = ZNUMA::memory_id(untype(ZOffset::address(start())));
      | ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/hotspot/share/gc/z/zPage.inline.hpp: In member function 'void ZPage::set_last_used()':
src/hotspot/share/gc/z/zPage.inline.hpp:210:20: warning: conversion from 'double' to 'uint64_t' {aka 'long unsigned int'} may change value [-Wfloat-conversion]
  210 | _last_used = ceil(os::elapsedTime());
      | ~~~~^~~~~~~~~~~~~~~~~~~
It might make sense to fix all the ZGC code at once.

9 src/hotspot/share/gc/z/zHeuristics.cpp
9 src/hotspot/share/gc/z/zDirector.cpp
8 src/hotspot/share/gc/z/c2/zBarrierSetC2.cpp
6 src/hotspot/share/gc/z/zStat.cpp
6 src/hotspot/share/gc/z/zNMethodTable.cpp
5 src/hotspot/share/gc/z/zStoreBarrierBuffer.cpp
3 src/hotspot/share/gc/z/zRelocationSetSelector.cpp
2 src/hotspot/share/gc/z/zUnmapper.cpp
2 src/hotspot/share/gc/z/zPageCache.cpp
2 src/hotspot/share/gc/z/zMetronome.cpp
2 src/hotspot/share/gc/z/zMarkStack.cpp
2 src/hotspot/os/linux/gc/z/zSyscall_linux.cpp
2 src/hotspot/os/linux/gc/z/zPhysicalMemoryBacking_linux.cpp
1 src/hotspot/share/gc/z/zVerify.cpp
1 src/hotspot/share/gc/z/zHeapIterator.cpp