neon/ext: work around bug in GCC prior to 9.0

Without this we end up with messages like this for each function.

  /tmp/cc2WM2zv.s: Assembler messages:
  /tmp/cc2WM2zv.s:47871: Error: operand mismatch -- `rev16 v0.4h,v1.4h'
  /tmp/cc2WM2zv.s:47871: Info:    did you mean this?
  /tmp/cc2WM2zv.s:47871: Info:          rev16 v0.8b, v1.8b
  /tmp/cc2WM2zv.s:47871: Info:    other valid variant(s):
  /tmp/cc2WM2zv.s:47871: Info:          rev16 v0.16b, v1.16b

simde-common.h

@@ -793,6 +793,9 @@ HEDLEY_DIAGNOSTIC_POP
 #    if !HEDLEY_GCC_VERSION_CHECK(9,0,0) && defined(SIMDE_ARCH_AARCH64)
 #      define SIMDE_BUG_GCC_ARM_SHIFT_SCALAR
 #    endif
+#    if !HEDLEY_GCC_VERSION_CHECK(9,0,0) && defined(SIMDE_ARCH_AARCH64)
+#      define SIMDE_BUG_GCC_BAD_VEXT_REV32
+#    endif
 #    if defined(SIMDE_ARCH_X86) && !defined(SIMDE_ARCH_AMD64)
 #      define SIMDE_BUG_GCC_94482
 #    endif