GCC implements extensions for C as described in the ARM C Language Extensions (ACLE) specification, which can be found at http://infocenter.arm.com/help/topic/com.arm.doc.ihi0053c/IHI0053C_acle_2_0.pdf.
As a part of ACLE, GCC implements extensions for Advanced SIMD as described in the ARM C Language Extensions Specification. The complete list of Advanced SIMD intrinsics can be found at http://infocenter.arm.com/help/topic/com.arm.doc.ihi0073a/IHI0073A_arm_neon_intrinsics_ref.pdf. The built-in intrinsics for the Advanced SIMD extension are available when NEON is enabled.
Currently, ARM and AArch64 back ends do not support ACLE 2.0 fully. Both back ends support CRC32 intrinsics from arm_acle.h. The ARM back end's 16-bit floating-point Advanced SIMD intrinsics currently comply to ACLE v1.1. AArch64's back end does not have support for 16-bit floating point Advanced SIMD intrinsics yet.
See ARM Options and AArch64 Options for more information on the availability of extensions.