MFH: r400578 (slightly reworded), r400697

multimedia/openh264: unbreak build on aarch64

/nxb-bin/usr/bin/cc -O2 -pipe -I/usr/local/include/gmp-api -fno-strict-aliasing -DHAVE_NEON_AARCH64 -fPIC -DGENERATED_VERSION_HEADER -I./codec/common/arm64/ -I./codec/api/svc -I./codec/common/inc -c -o codec/common/arm64/copy_mb_aarch64_neon.o codec/common/arm64/copy_mb_aarch64_neon.S
<instantiation>:4:1: error: unknown directive
.func WelsCopy8x8_AArch64_neon
^
codec/common/arm64/copy_mb_aarch64_neon.S:186:1: note: while in macro instantiation
WELS_ASM_AARCH64_FUNC_BEGIN WelsCopy8x8_AArch64_neon
^
<instantiation>:2:1: error: unknown directive
.endfunc
^
codec/common/arm64/copy_mb_aarch64_neon.S:196:1: note: while in macro instantiation
WELS_ASM_AARCH64_FUNC_END
^
[...]

Backout r394430 as armv6 can do NEON on some CPUs but runtime
detection on FreeBSD is unavailable. aarch64 always has NEON.
So, detect NEON via compiler defines:

1. make.conf: force NEON for armv6 architecture (newer boards) CFLAGS+= -march=armv7-a

PR: 201273 (ping)
Approved by: ports-secteam (feld)