target/arm: Allow use of upper 32 bits of TBFLAG_A64

target/arm: Allow use of upper 32 bits of TBFLAG_A64

The TBFLAG_A64 TB flag bits go in flags2, which for AArch64 guests
we know is 64 bits. However at the moment we use FIELD_EX32() and
FIELD_DP32() to read and write these bits, which only works for
bits 0 to 31. Since we're about to add a flag that uses bit 32,
switch to FIELD_EX64() and FIELD_DP64() so that this will work.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Miguel Luis <miguel.luis@oracle.com>