neon: avoid warnings when "__ARM_NEON_FP" is not defined.

[clopez]

WebKit project recently imported simde 0.8.2 Since then the build for Aarch64 targeting RPi4 boards has started giving lot of warnings related to the simde header:

warning: "__ARM_NEON_FP" is not defined, evaluates to 0 [-Wundef]

Add a check to ensure that __ARM_NEON_FP is defined.

Related: https://bugs.webkit.org/show_bug.cgi?id=273789