From declarations of bit() member functions (for structures Register,
FPURegister and FPUControlRegister) are removed constexpr specificators.

Build V8 will fail if function bit() is declared as a constant
expression.

TEST=
BUG=

Review-Url: https://codereview.chromium.org/2737143002
Cr-Commit-Position: refs/heads/master@{#43674}