Current safepoint poll data reside all way down in JavaThread, which makes the encoding for safepoint polls not as efficient as it could be, especially on x86. We can pull it closer to the rest of nmethod/gc-related hot fields.

Current encoding:
 0x...6d17: mov 0x450(%r15),%rax
 0x...6d1e: test %eax,(%rax)
 0x...6d20: (+9 bytes)

Encoding with lower offset:
 0x...1018: mov 0x30(%r15),%rax
 0x...101c: test %eax,(%rax)
 0x...101e: (+6 bytes)