Fix ARM NEON support detection on ARMv6 target
* configure.ac (gcry_cv_gcc_inline_asm_neon): Use '.arm' directive instead of '.thumb'.
Fix allows building ARM NEON assembly implementations when compiler
target is ARMv6. This enables NEON implementations on ARMv7+NEON CPUs
running on ARMv6 OS (for example, Raspbian on Raspberry Pi 2/3).
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>