[builtins] Fix value of ARM_INEXACT

The existing value of 0x1000 sets the IXE bit (Inexact floating-point exception
trap enable), but we really want to be setting IXC, bit 4:
Inexact cumulative floating-point exception bit. This bit is set to 1 to
indicate that the Inexact floating-point exception has occurred since 0 was
last written to this bit.

Reviewed By: kongyi, peter.smith
Differential Revision: https://reviews.llvm.org/D98353