[Lsan] Use fp registers to search for pointers

X86 can use xmm registers for pointers operations. e.g. for std::swap.
I don't know yet if it's possible on other platforms.

NT_X86_XSTATE includes all registers from NT_FPREGSET so
the latter used only if the former is not available. I am not sure how
reasonable to expect that but LLD has such fallback in
NativeRegisterContextLinux_x86_64::ReadFPR.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D87754